Parameter for Units in a Model

I'm trying to create a relations in my model that is determined by whether it's metric or inch. I've been unable to find the parameter that reports if the model is metric or inch. I have a call into PTC, but they have not called me back, so I'm turning to the experts. Is there a parameter in the model that states the units of the model, if not is there a way to get that in other ways?

Thanks in advance,

Mike Heath