If you have a TMobile phone that you use on TMobile, you will notice absolutely no difference after you unlock it. It will function exactly the same before and after. No additional capabilities on TMO. That's it. Whatever you could do before, you'll be able to do afterwards. No more, no less.
The only way you will even know for sure it is unlocked is through the menus.
Go to Options >Advanced Options >Sim Card. Type mepd. You won't see any letters being typed but it will open a new screen with a list on it. Look for Network. It should say "inactive" or "disabled" if the phone is unlocked.
The only other way you will know it's unlocked is that it will work with a SIM from another GSM provider such as AT&T.