Hold your bike off of the ground (or flip it upside down). Look at the disc brake. Look into the caliper and see if there is space between each pad and the rotor. Sometimes it helps to shine a flashlight up from the other side of the caliper. If you don't see space on one side of the rotor, your caliper is misaligned. If you do, spin the wheel. Does the rotor move back and forth at all? If yes, then your rotor is bent.

Whatever you do, if you have hydraulic brakes, don't squeeze them when the bike is upside down. It can put air into the system which can temporarily make the brakes not work well.