How can you tell if the muscle & skin have re-attached internally after a tummy tuck?

Healing is not complete after 3 weeks.  The sensations you are experiencing sound normal.  If you are concerned about your healing then I would revisit with your plastic surgeon. 

Generally if the abdominal draped is not adhere to the abdominal wall a seroma forms. This is something that will require an examination by your plastic surgeon.

I am sorry to hear about the concerns you have, but your description does not sound alarming to me. Generally speaking, many patients ( and their plastic surgeons alike) tend to underestimate the true recovery time necessary after tummy tuck surgery. Many patients experience significant physical and emotional “ups and downs” after this extensive surgery.For your own peace of mind, I would suggest that you follow up with your plastic surgeon who will be in the best position to evaluate your concerns, provide meaningful advice and/or reassurance. Best wishes; hopefully, you will be pleased with the longer-term outcome of the procedure performed.