What are they gift cards for? If some of them are for a retailer where you've got a local branch nearby, you could take them in and ask them to scan them or, if they're not the type with magnetic strips, to find another way tell you whether they're still valid .
I've found really old gift cards at the back of a drawer, for stores like Debenhams, and have never experienced any problems with checking on their validities.
Some gift cards remain valid indefinitely but nowadays many expire within a fixed period (which is commonly either 12 or 24 months). All you can do is to ask the card issuers about the ones you've got.