Believe it or not, there are plenty of professionals out there with significant admin experience with both Unix and Windows. Being a Windows professional doesn't make you some sort of dirt-eating Tauron, nor does it necessarily make you a "fan" who's chosen his side in some nerd-rage fight to the death.
Most definitely. I was a server admin for clients of mine who were too small to have one full time. Ran Linux on my own desktop, also had Windows and Linux servers running on different machines. I could deal with either. I wasn't a "fan" of anything. I was a professional who took care of my clients. Unix, Linux, Windows, it didn't matter. What mattered was my knowledge and making whatever they had chosen to run work. And work well.
As far as how to gauge their skills.... You won't be able to, as the good ones will know more than you do. Pay attention to what they have done in the past, contact their previous employers. Certs don't mean much, I've run across a few that didn't know anymore than was needed to pass the tests.
Maybe pose problems in a Linux domain that you are familiar with, ask them how they would handle that in Windows. Ask them to explain how it works differently from what you're doing. Ask them general security questions that should be known by all server admins. Firewalls, etc.
If you're in charge, you need to be able to assess their work. And that depends on the type of Win servers you're going to run. Outward facing? Database? In-house application only?
So much depends on what the servers do. Someone may be a great domain admin, but suck at the database side of things.
I know, not much help. But please don't call them "fans" :)