Okay I have a 1997 ford 250 7.3 When im driving a

Okay I have a 1997 ford 250 7.3 When im driving a dont hear the turbo whistling when I get on it. But when Im parked in the drive way and I rev the engine I think I hear the turbo kicking in. My fuel mileage has been sucking The turbo has been replaced and recently had the fuel leak underneath the turbo fixed. Can anyone give me som advice on what I can do to see if the turbo is working correctly. Please email me at drewmgd73@yahoo.com