While you can't check the water, you can have them run through everything they did to winterize it and, also, explain what to do to unwinterize it. (If you're not familiar with that stuff.) If you're new, bring a notepad and take notes and also take pictures.
(A tip from my dealer was to take and print out, in color, pictures of all of the outside labels as the sun can make them fade and it's good to have them...especially the specifications, model info, etc.)
A realtor friend gave us this tip years ago...when doing a walkthrough, have a 3-prong tester to check EVERY outlet. (I did this on a new house and actually found 2 outlets that weren't working or wired wrong.)
I think there's also many great tips in here on walk-throughs....don't let them rush you and be asked to be showed (not just talked about) everything that they can.
Have fun...excited for you!