Distinguish Between Users and Service Accounts in Active Directory

I've not seen anything that could be construed as an 'official' standard. What I've typically done is used a standard naming prefix as well as keeping them in an OU. You could use the Description field or the Department field as well for an easy sort/select.

There is no "official" solution to this issue, nor any specific AD attribute meant to convey "this is a service account". Various places uses various techniques, which may include OUs, groups, descriptions, name prefixes, and so on; but it really is only a cosmetic distinction: service accounts are the exact same objects as user accounts.