No not really. It's something you need to develop a feel for through experience and observation. There are clues:
*) The Poster: Does he pay full price for partial orbits?
*) The habitual limper: Is he the first player in for just the price of a BB? Does he do this from late positions after everyone has folded to him? It's especially bad if he open limps from the button. Open limping is bad for a number of reasons: you give up all your fold equity, you failed to take the initiative, if you have a limping range, you need to protect it by limping strong hands, and this costs you EV.
*) Bad bet sizing: Do you see min-bets that offer the rest of the table high odds? Shows he doesn't understand odds, and/or bluffs gutlessly. Does he make these tiny bets with strong hands that leave value on the table? Larger than game standard opens? Indicates he's afraid of post-flop play.
*) Plays the same hands the same way regardless of position: doesn't understand the power of position.
*) Short buys: Does he come in for less than a full stack? Especially: does he repeatedly buy in short after losing a short stack? Could indicate he's afraid of losing too much, so what does that say about his confidence? This often costs him more in rebuys than he'd lose coming in for a full stack. (Note: Beware the player who comes in for a short stack, takes a beat and makes a full rebuy, and/or tops off later. This could be a better player assessing the fishiness of the table.)
These are all reliable indicators that your opponent doesn't know what he's doing.
Excellent observations, would just like to add a few addtional points.
*) The poster: If a known maniac is at the table, it is often worth posting from the CO seat especially at full ring to not miss out on a golden opportunity. Good players will sometimes do that.
*) The limper: I love attacking limpers, but when a TAG or NIT suddenly start to limp, things get a bit weird. Are they doing the limp-trap with aces, or are they trying to setmine cheap? Some caution is required in this situation, and I am not looking to get involved with really marginal hands.
*) Bad bet sizing: Or on the turn they bet 10c into a pot of 1,65$, but suddenly on the river out of the blue they overbet 2,7 times the pot on a blank. Just let them have it and move on.
*) Short buys: A very reliable tell is odd buyin like 6,83$ on a 10NL table, when someone sit down. Pretty much the only reason, someone would buy in for 6,83$, is because, this is their entire remaining balance. They lost the majority of their rool, and now they are looking for a quick bingo style fix.
*) Donk betting: Betting into the previous street aggressor is not always bad, but bad players tend to do it way to often, and in bad situations. If you open UTG, SB call you, flop come AK4 rainbow, then you are the one representing most of the good hands on this board like AA, KK, AK, AQ. So leading into that is just strange and not something, a good player would ever do.
