There isn't an "expiration date" on candy melts, but I know that sometimes it just doesn't work right (separates, etc.) and I think that it might be because they're old. How do I tell when they're too old? I have a collection of different colors, and hate to just throw everything away...
I've never, ever, thrown any away. Yes, they can bloom, but once heated, I don't have a problem with them.
I melt all of mine in the microwave in a silicone bowl, 50% power, 30-45 seconds, stir, repeat, until 90% are melted, and then stir until they're all melted.
If they seem too thick, I add paramount crystals or very little bit of vegetable oil or shortening.
If you email Wilton, they can tell you. I forget how they code the package, but a poster on the Wilton site emailed them and they told her. The Wilton forum search is about as good as the one on here, but you could try there as well.
LIke Rae, I've never thrown any away either, and I have ended up with some weird colors. i have dark green now. I user Merkins, not Wilton, and they don't seem to ahve a date on them. But I agree that just reheating them should work. I've never heard of chocolate truly "going bad," though I'm sure at some point it must.
At some point , they can be bad. I use wilton and merkens. especially, the white, can change. A friend of mine, bought me 5 lbs. of merkens white. she told me she thought they were old. so we tested, they just wouldn,t melt(didn,t want to waste the paramont crystals in them). they just didn,t smell good either. so you will know if and when to throw out. wilton or merkens.hth