Check if a SQL Login exists without SA How can I check if a SQL Login exists for an arbitrary domain user if the account I'm running doesn't have SA rights to the instance (but will be dbo on at least one DB)

Ideally I'd like the solution to be automate-able for inclusion into a utility at some point in the future.

If I create a user in my DB and attempt to map it to a SQL Login that doesn't exists will it report that it doesn't exist or just give me a permissions error?
SQL Server will allow you to create a database user for a domain account, even if the corresponding login does not exist. No error will be returned as long as the user creating the database user has CREATE USER permissions and the domain account is valid. 

However, the new user will not be able to authenticate to the instance until a login for that account (or group the account is a member of) is created.