For an owned system, ask for the installation cost, in-service date and production history. A home inspection, typically $400 to $600, should also confirm the roof under the panels is sound, since system age and size affect how much weight that documentation carries.
For a leased system, your agent should confirm whether the lease can be assumed, what the remaining term and payments look like, and whether your lender is comfortable financing a home with an active lease obligation attached, since some are reluctant.
Either way, get the paperwork before the offer, not after, because a lease you can't assume can turn into a condition to remove the panels or renegotiate. Get your hand-reviewed valuation from Joel Dyck.