Bank account is a good indicator I think. If a team has more cash than his whole team value is worth, chances are he didn´t ran out of tokens but just doesn´t play. Also think about transfer market limits. If a team has a level 37 player without special abilities in season 4 he most likely must have logged in at least once in season 4.
In your case, if he has lots of money but a poor team he´s most likely inactive. If you´re really curious about it, screenshot his formation and see if he ever changes it.