If your parent's investments are made through a brokerage (eTrade, Charles Schwab, etc) they will get a statement (sometimes online) every month that tells them the value of their portfolio. Same for 401Ks through work, and most any other US based investments.
If they own property besides the house they live in (which is exempt) it's a little more complicated, but they should get a asseessment notice with their property tax bill every year that tells them what the state gov't says the property is worth. This works in most states but CA, where we have messed up limits on reassesments. If the property tax assessment is artificially low (you live in CA or a similar state), just make your best guess, and because they financial aid office won't be able to prove that if they sold the property, they could get X dollars, its okay to estimate a little low here (no where else).
Estimates are okay, but you may get asked for verification, and if they find that you estimated lower values for investments, you will have to verify every year (which is a pain) so make them as accurate as possible.
I do this every year with my father, it usually takes us two hours to find all the paperwork necessary to fill it out. And there is nothing on there that requires going to an accountant.
