Forks get bent from running into things or from being run into (or over). Typically, both blades bend because they are joined by the crown at the top and by the hub at the bottom. Generally, forks get bent back or to one side. sometimes only one will bend, usually backwards, and put some twist into the other. Steel is pretty elastic, which means it likes to spring back to its original shape. Therefore, it has to be moved a lot to stay in its new position, i.e. to get bent. This means that most bent forks look bent.

If a bike veers off to one side when riding no hands, the fork is probably bent. If you can ride no hands and the bike tracks straight, the fork is either not bent, or both blades are bent back the same amount. As long as the bend is in the blade, there is a good chance of straightening the fork. If the bend is between the crown and steerer tube, the fork is probably junk. You can usually tell this by how the headset feels: if there is no play or binding when rotating the fork in the frame, the steerer is probably OK.

A fork that is bent back generally looks like it. A fork that is bent to one side generally feels like it. Basically, if you cannot see or feel a bend in the fork, it is likely not bent, or not bent enough to matter.

By the way, the classic method for straightening a fork that has been bent back is to reverse the fork with a wheel installed, then wheel it hard into a wall. I am not recommending this, but in the proper hands it works surprisingly often and surprisingly well.