my car was overheating and i got water steam coming out the tail pipe

so i changed my head gasket to a brand new one and i just finished the job

anyhow now the car stilll overheats but no smoke comes out the tailpipe. i think my water pump is bad but i dont know how to check it. The car will overheat after about 15 or 20min. its funny too because the fans on the radiator are not coming on when it does overheat but when i turn off the engine the fans turn on. This is awww annoying

somebody give me some advice