You CAN put the old plugs back in, although that's less than desirable. If it runs well, one plug is bad. Just because the plugs are new, it doesn't mean they're good--as sad as that is. It happens occasionally.

If the engine runs fine, it should be easy to isolate which one of the plugs that you took OUT is the bad one. Keep track as you remove them with respect to what cylinder they came from. That way, if it's a bad spark (leakage from a wire) you'll have a good idea which one.

It's a simple way to diagnose it, and not very time consuming.