I have an area that is only covered by T Mobile and would like to know if they are a roaming partner of AT&T.
That's certainly a possibility. The best way to check would be to http://www.att.com/maps/wireless-coverage.html and zoom in on the area in question to see if it shows as no coverage or as partner coverage. Keep in mind that roaming the majority of the time will likely prompt AT&T to discontinue your account, so make sure the majority of your usage will not be on a partner network.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.