The most basic information – keep them in cool, dry and dark conditions ideally..
Seeds that have been stashed in tin on a shelf in a shed might be ok if you can guarantee that these conditions have been met.
Seeds stashed in a drawer in the kitchen might not fare so well… as it can get humid in a kitchen.
As for seeds left in a conservatory or greenhouse – mmm not sure these will still be viable – sorry 😦
This might seem harsh, but a seed swap works best on trust. If someone takes home a packet that has been poorly stored, they are probably not going to have much luck…
We’ll tackle saving seed in another post 🙂 Meanwhile here is a link to a handy chart so that you can check your seed packets in advance 🙂