Is my iphone 3gs locked to att?

I currently use H2o wireless on my iphone previously used att prepaid.
Bought as a unlocked att service used phone.
I would like to be able to use a tmobile sim on it?
I keep reading that they share towers and on the same frequencys…
Extra Info in my phone
Settings>General>Network H2o
Carrier AT&T 14.0

Well if you are using another carrier on an AT&T originally iPhone then it is unlocked.