What bcr said pretty much sums it up. The problem is some signs won't show immediately, which is why you wan't to quarantine for a minimum of 90 days to be safe. You have to think about it like this, is one snake worth wiping out your whole collection? I QT mine, and I also treat for mites even if I don't see any, more so as a preventative measure. One of the first thing I do with all new additions is I soak them, just in case they're dehydrated, help them defecate, and I'm able to check for mites a bit easier.