Trained eye can catch it on a tach. Accelerate slightly when you think its locked.... if rpms do not rise its locked up.
Also steady speed slight acceleration lightly depress the brake petal & see if rpms jump 3-400 rpm. It should jump... although if the clutch slips more than 200 rpm it will throw a dtc.
Most scan tools show rpm of tcc slipage.
I might mention that in a '96 chevy w/ 4l60, I am up to speed about 60-70 mph, tap the brake there is to be a small rise on rpm. Then let off the brake, rpm drops and it's normal. That's how I understand it.