How can you tell if something has been defrosted and then frozen again?

[devilspixie]

I'm a little concerned our freezer isn't working properly - I have noticed ice crystals in bags of frozen veg and in a layer on top of, for example, those frozen mousses you get. The stuff is frozen, I was just concerned it had maybe thawed and then re - froze. There hasn't been a power cut and the door hasn't been left open or anything, its just something I've noticed a couple of times now (and I have a salmon in there just now and am concerned its been ruined!!)