How to tell if a rat is a good rat when you get them from the pet store?

I'm planning on getting rats soon but I only have a pet store by me and no breeders. I know when you get them from a pet store you don't no their background or they could be mean or sick but, when I'm choosing the rat and holding it will it show if it's sick or mean? Like will it bite me right of the bat when I hold them? Also the pet store I would buy them at is a private owned pet store called Blue Ridge Pets unlike Pet Smart or Petco, and I don't know if they breed thier own animals (which I'm pretty sure they do) but, I do know that they don't just focus on breeding rats and rodents. Would tha pet store be different from other world wide pet stores like Pet Smart and Petco? Thanks for all the help! One more thing... when you bathe rats do you use regular soap or shampoo, just water, or will they just have something at the pet store I can use?