Bad mice?

How do you tell if a frozen mouse (or rat) has gone south and shouldn't be fed to your snake? Is it just funky smell or are there visual signs I should be on the lookout for? Also, is refreezing uneaten rodents advisable or would it lead to spoilage?

The skin tends to get really thin like it looks like they might pop, smell is also bad, or swollen bellies (gas). I will defrost in the fridge, warm up, offer and if they don't take it I will refreeze a rat or mouse *once*. I throw it away after the second offering if it's still refused.

Thanks, I'll definitely keep those signs in mind! And that's what I thought too with the refreezing, not more than once for the same rodent, and probably not if it's been room temp or higher for more than an hour or so.