When you see a player is just calling (limping) into a pot you need to be suspicious. This is especially true if they are usually an aggressive bettor. Sometime they are on a draw but they may also have the nuts and just be letting you do the betting for them. If they were in the big blind they may have gotten a big blind special so pay attention and take a little extra time to try and figure out what they would be calling you with. Don’t keep betting into them blindly.