Start by going to t-mobile.com and checking the coverage maps. However, coverage with any carrier is EXTREMELY location-dependent. Coverage maps can only give approximate coverage areas. Signal strength within a coverage area can come and go from mile to mile, block to block or even room to room.
People are constantly asking about coverage but trying to give meaningful advice about expected coverage is almost worthless. It is. The only thing that really works is first, check coverage maps to get a rough idea what to expect. Pick a provider that shows good coverage in your area. Then actually try their service in places you expect to use it. Walk around the house, work, etc and see if it drops calls or loses data connection.
It's the only way to be sure you'll be happy with any provider's signal quality.
Registered Linux user #266531 since 2001. Android user since v1.0.