I just cracked one open out of a case I bought about 5 years ago and lit it up. Seems to look normal and be working fine. I'll see how long it goes-usually I get about 3 hours out of a new can. If it runs a significantly less amount of time then I'll know there has been some evaporation.

Edit: it just went out. It burned for 2 hours and 50 minutes, so it looks like it stored pretty well.