yes carry out all the possible combinations so for a two input gate that is 00, 01, 10, 11 and see if the out put is correct for each state. just make sure u r injecting the apropriate signal for that type of gate and if u r injecting it externally and not testing the signal already present then i advise u to remove the chip from the circuit (these should be pluged into sockets so that should not be a problem) maybe using a bread board or standalone socket with wires to it will help u in the testing.