Rats: look for clear eyes (no reddish or brownish goop around the eyes.) A nice rounded shape without the rat being overly fat or underweight skinny. A female with a pear-shape might be pregnant.. this might make her too large around for your snake. Also, make sure that the rats are not missing patches of fur or having scabs on the skin. Whole, clean tails. Active and robust "feeder" rats, which are not regularly handled, are generally afraid when someone reaches for them; a rat that just cowers or is breating very heavy and rapidly is probably ill.