Spotting a bluff isn't easy or everyone would be into poker
Even the pros aren't ever 100% correct with sensing bluffs; we can be more confident in the opponent bluffing or not, but we can never really be sure until the cards are revealed.
I'd say the two biggest things to look for are:
1) VPIP
and
2) Hand Range funnel during the hand
The first one is more simple. VPIP is "Volunteer Put In Pot" statistic and this shows how often they are playing hands preflop and seeing the Flop with similar statistics measuring similar things. The larger the VPIP percentage, then the greater the chance this player bluffs more often.
If a TAG player has a VPIP of 10%, then this TAG (close to NIT with this lower percentage) is mostly waiting for the good cards and will be bluffing much less often if ever. Conversely, a maniac with 50% VPIP is getting to the Flop with literally 50% of all hands dealt. They might be in there with more speculative hands like A9o or 98s in addition to the good cards like AA. Someone playing this many hands has to be bluffing more often if they have a hand like 98s on a Flop of
Ah Kd 3c
The VPIP percentage that is more "normal" varies on the player play-style and factors like the table they are playing at (how opponents are playing). Poker has many play-styles that could be profitable, but generally a VPIP between 10-25% is more "normal." The lower percent closer to 10% are tighter players who wait for good cards more and the higher percent players closer to 25% are looser players who bluff more. Someone with 40% VPIP or higher is almost certainly a recreational player who is playing a losing strategy unless it is an extreme situation where the table is letting them run over everyone because 40%+ is most certainly bluffing a massive amount and probably not "good bluffing."
The second one is much tougher to grasp. This takes a lot more experience because it requires post-flop skill and hand reading. Basically, this means that your opponent is more likely to be bluffing when their "story" isn't adding up. This is easier to explain with an example.
Let us imagine a full table of 9 poker players. The player in Middle Position (MP) open-raises the 3x the Big Blind (if blinds are 50/100, then they raise to 300) and we call on the Button (BTN). Literally everyone else in the hand folded, so now the two of us see a Flop.
Notice that so far I didn't tell you what cards the opponent has or what cards we have. Our goal is to try and narrow down what cards the opponent likely has and our cards aren't usually too important in figuring this out, but we might come back to this later.
Okay, so let us now see the Flop:
8d 4h 2c
Our opponent bets half pot (c-bet) and we call.
The Turn card is:
2s
Opponent checks and we check.
River is 7h and the opponent checks, so we decide to...
How to figure out what they likely hold here. It greatly depends on the opponent play-style. Are they a pro who adapts often? Are they a tight regular? Are they a loose and recreational player? All of these players might mean different things.
Breaking down this example: we might have arguably played the hand differently, but we are only going by how this was played. When the opponent open-raises preflop, this means that they tend to have a stronger hand they want to play. A TAG or Nit might be raising with hands like AA or KK but a LAG player might also be raising with 55 or 98s. It depends on the player, but since they open-raised here, let us eliminate a lot of dubious hands like 72o or Q3s. There are many hand combinations we can eliminate and some of these might prove important later and some might not.
For whatever reason, let us say we believe they are likely only raising preflop these hands: AA, KK, QQ, JJ, TT, 99, AK, AQ and folding the rest or limping in with the rest. This might not be the best strategy and this very Nitty open-raising range is for sure simplistic and exploitable but let us just use this small range of hands to make this example easier.
Okay so now the Flop comes down:
8d 4h 2c
When they continuation bet and we call, we have to narrow down the range of hands we believe they now have. Could this player be holding Two Pair Eights and Twos? No. We eliminated this combination preflop when they open-raised, remember? Might they hold Top Set with Pocket Eights here? If our range we gave the was exact then no, because we had them continuing only with 88+ pocket pairs by them open-raising.
You'll notice already that we don't know what they have but we know a lot of combinations they likely do NOT have
At this point in the hand, our opponent likely has AK or AQ and missed the Flop. Yes, they might have an over-pair like KK, but there are only 6 possible hand combinations of KK and there are 16 possible hand combinations for non-pocket pairs. This is where a little hand range study and experience comes into play.
Remember, I never said what cards we have in this hand. What if instead of a c-bet, the opponent just shoves All-In on the Flop. This is the time where we would have to decide if they are bluffing or not. If our range is correct, then they probably didn't pair any of the low cards on the 8-high board. They are bluffing with a missed AK/AQ or hold a single overpair. If we Flopped Two Pair or better on this Flop, then we can usually call down this All-in bluff confidently
Remember, their "story" didn't add up. What were they representing with the All-In? Three of a kind in 888? Maybe, but would they open-raise with 88? Some players might and some would fold it preflop or limp in to see a cheap Flop and try to hit a set. This is why the opponent play-style matters.
If they shoved All-in here, then they are almost always bluffing
Continuing with this example though, then Turn was 2s and both players check. This Deuce (2) doesn't really change much to the board texture. It is another low card unlikely for any of us to be holding and especially unlikely for them to be holding since they open-raised. Since this Turn card changes the board texture and range little, we call this card a "blank." It holds little importance. If we were ahead, then we probably still are and if we were behind then we probably still are.
River: 7h and the opponent passively checks.
What do they have by now? Their "story" is consistent with AK or AQ (or a high-unpaired card if our range was off) that missed. They open-raised preflop, missed but c-bet, then gave up on the hand. Could they hold the Straight with 8-7-6-5-4 by holding a hand like 65s? Unlikely. Our opponent would of had to open-raised this hand and we eliminated this option earlier in the hand so they can't hold something we already eliminated from their range (assuming our range for them is correct).
If we actually had a weak card paired such as Ad4d pairing the 4 on the Flop, then I think we have a clear value bet on the River. If we have a 2 somehow like A2o, then our Three of a kind can even beat their overpair (although I leaning towards them not having an overpair like AA since they didn't show more aggression in the hand and passively checked the River).
I hope this long example helps illustrate a little by what I mean by using their range of likely holdings to more accurately determine if they are likely to be bluffing or not based on how the hand played out