if it's a private rental party then safety is your first priority. watch for barrel condoms and masks most importantly. explain all the safety rules, know them by memory. you will get a lot of questions so be prepared to answer them. enforce all the safety rules, but don't be a dick about it. remember, they're there to have fun, not play competitive paintball. rental parties are the people you want to have a great time and come back to your field. rental parties are the ones that bring in the money for the field because they have no equipment and must more money at the field for the rental package.
i highly doubt you'll have to watch for cheating too much. most first-timers don't like getting hit so they'll probably call out because of the sting of being hit. still though, i've seen it happen. just don't be a dick about it and remind them of the rules and enforce them as instructed by the manager of the field or the head ref.
and you're going to get shot. especially if it's in the woods. every time i ref in the woods i'll be walking along to another area and some trigger happy player will by lying in a bush ready to shoot anything that walks by. of course i'm the guy that walks by and ends up starting down the barrel of a tippmann 98 rental with a customer screaming "SURRENDER SURRENDER SURRENDER!!!!!" and they shoot me anyways.
it happens, lol.