If it's been used then most of the systems should be good. The things that will be bad are the tires and maybe some parts of the suspension, brakes, bearings and stuff that is not intended to be sitting still.
The tires are probably aged out and you should just get new ones, including the spare. Have the brakes checked (hey shouldn't be too bad if they weren't used) and the wheel bearings checked, cleaned and repacked (or just replaced).
Have a PDI with the owner before taking it over. A Pre-delivery means you run and test ALL the systems (more than just turn it on) to ensure all the plumbing, appliances, A/C, electrical, etc. It's like a walkthrough for a house but you're doing it on a trailer. This includes stabilizers and a look under the rig.
Unfortunately as a newbie you may not know what to look for. Find a knowledgeable friend that might be willing to help.