like VANQUISH said sometimes you just know. How do you just know, well you've gotta be playing alot of attention to who your trying to get the bluff read on.
First if you feel like your constantly getting run over with medium hands than your probably not raising enough. "To the aggressor goes the spoils". If your just letting people lead into you, you'll be second guessing your self on the next street everytime if your unsure.
as far as getting bluffed. You have to think about position and how many people are in the hand. than you have to consider the type of players, and you have to consider if that person thinks that you are weak. You have to think do I look like I am playing weak? Because in this case they will most likely take their chances trying to take the pot from you. Also You gotta think about what they can call with preflop and whether the flop is likely to have hit them.
It really just comes with time and hand experience though man, you have a lot to play a lot of hands, Its hard to teach reads. You gotta learn the betting patterns and pay attention to every showdown, gotta call alot when your getting pot odds
, even when you know your beat. Sometimes when I'm getting the right price to call I'll call just to see what the player could be giving me these odds with so I know what rangeof hands he will do this with. Thats a mistake I see alot of people make, they'll call all the way down to the river, know they are beat on the river, but when they get bet in to and are layed like 10 to 1 odds to call a somewhat small sized pot they'll fold. AT THAT POINT YOU SHOULD CALL JUST TO SEE WHAT THEY ARE BETTING WITH. You may see something that will make you say "hey, if I check-raised the flop and took control I could've probably won this hand with nothing"
Once again tho, in online poker its not signs, its being able to put your opponents on hands during and before the hand. Also realizing the competency of your opponent and who is able to make the deceptive plays and who isn't. Also you have to realize, that the other person HAS TO BET to win the pot, if he doesn't bet he'll probably lose to you if you have something. I wouldn't get caught up in that overbet mess tho, becauuse often times people overbet the pot out of nowhere with the nuts or might have hit a set on the next street and don't know how to extract the value correctly for the hand.