This problem is somewhat the opposite, because in poker you are never sure of anything, your opponents can make a lot of traps, so in my opinion I would say that there is no clear sign of a bluff. Now my advice is not to bluff too much, and a good bluff is one where you don't run as much risk of losing a lot of chips or know that your opponent doesn't have much game, or you can also semi-bluff when you're close to making a game, I would say it's the best option
There are no direct signs of bluffing. It is always risky to show if you do not have the highest hand on the table. If you are just starting to play poker it is better not to bluff at first or very little.
The most common sign in my experience is when you connect with a very wet board and you are betting and called down on the flop and turn, then your opponent checks/raises you on the river or bets if you check while no draws were completed.
In my opinion, players who like to play with junk often bluff. Observe the game, the behavior of the players at the table, and thanks to this you will better understand in what situations your opponent is bluffing.