The SPUser class has an IsSiteAdmin property. That may give you what
you're looking for.

There is a 3rd product called Universal SharePoint Manager that may
allows you to do this. here is a scrprnt of a few of the security
reports available:

Security Level Reports
User Permissions/Access Report
View a user  or groups permissions  and access across the entire
environment.
Group and Role Permissions/Access Report
View a specific group or role permissions and access across the
entire environment.
Site Security Audit Report
View the current security configuration of a specific WSS site.

Anonymous site  Access Report
View all the current WSS sites that have anonymous access
turned on.

Also checkout PSN Vulture, a free source tool @
http://www.opencanal.com/community.

I know of this product, but the price is very high and I only need it
for this purpose only.

I have custom C# code that lists each users  name and every role that user
has on the current site  in the format:

User name - role

is there not a 30 day version ????????

My old WSSUserUtil/SPUserUtil has an analyze feature that I think will
get you what you need.

I don't promote it much anymore, because I won't be updating it, but see
the following:

SharePoint Account Management using SPUserUtil - Part 3 - Auditing
Accounts

blogs.msdn.com/.../604021.aspx