This is what our company does, when for whatever reason going through the travel office isn't possible. And we're very large, with giant travel spend.

My guess would be that your two primary options for validating that across airlines are to go through a corporate travel agent or require the boarding passes. The latter isn't foolproof, but takes a lot of effort to get around.

Specific to United, you could look into this: http://www.united.com/web/en-us/cont...ness/uatp.aspx I'm sure a quick call to them would tell you if that had that capability.