Private aircraft insurance varies widely. You need to check the policy and see what it says. You must either be named on the policy, or covered by a blanket clause that says something like 'All pilots with the permission of the policyholder' or all pilots with more than 100 hours total time logged' etc.
Take the opportunity to check all the other documents as well, as private owners may not be so careful to keep things up to date.
MJ
