Well you didn't give any details about your environment, but assuming a Windows domain, why not Powershell?
Get-ADPrincipalGroupMembership someUser | Select Name
Now you have a list of all groups of which that user is a member. Of course that only applies to your AD domain, so it will not tell you if that user is a member of any local groups on a computer.
Also I notice you listed network-share as a tag of this question. If this a network share in question, be aware that there are two different kinds of permissions on a network share - share permissions and NTFS permissions. NTFS permissions have the final say, so configuring a network share as "Everyone - Full Control" on the network share permissions is pretty common, and then relying on NTFS permissions to actually grant or deny access appropriately.