i can't help you with the states. Though i think it varies by rat club and there's a couple major ones around.
Here in the uk we have a national club (the nfrs) which also acts as the main standards body too. so they set and agree the standards for each variety. Then there area a number of national clubs which affiliate with the nfrs if they choose to and so use the standards. The nfrs trends to run shows in areas not well served by affiliated clubs worth a good member presence. Round me mossy shows are local club shows and I'm pay off the committee of one of those clubs. I'm not sure if it works the same in the us but i do know the afrma is a fairly major club. Our nfrs holds a list of shows including local ones
In terms of standards i can link you to the nfrs standards but unless you are in the UK that won't be much help. It's worth looking at how the standard is set out and weighted when you decide if you want to enter a rat. Some things matter more than others. You should also look at the level of show your entering. Here we have local shows all the way up to championship shows. i would happily enter a rat i knew was a bit of the standard and unlikely to do well in a local show (for the critique) but in a championship show i would stick with rats i knew to be good.
One thing that matters a lot woth showing though is that your rats enjoy it, even a beautiful rat who is miserable at the show won't show them selves of well and most judges ate influenced by this. a stressed rat is also more susceptible to infections
It's also woth noting that shows do carry a risk of spreading infections.