This would differ from person to person.
If I had to generalize it I would say you are playing on tilt when "You are making decisions with no sound reason."
Being on tilt can also be physical:
Is your heart rate increasing?
Do you feel like you are breathing harder?
Are you angry?
Do you emotionally feel like you are a yo-yo?
The best thing an individual can do is to examine the times they are in tilt.
Ask the 5 whys you were in tilt. What did you feel like while in tilt? What happened once in tilt? What got you out of it?
Once you can recognize what gets you into tilt you can avoid those items. For instance bad beats happen and If they out you in a tilt state how can you avoid that? Would it help if you accept that a bad beat is part of the game and it is just probability.
If you cannot avoid something that will put you in a tilt state maybe you can developer a technique to help once you are in tilt. For instance if it is a bad beat then take a hand or 2 off. Find something to take you out of tilt like breathing slowly, getting some water.
Try to keep notes on what brings you into tilt, what happened once there, what did you do to get out if tilt. Review it often. Look at it and try to make small changes too get yourself out of being in tilt and help avoid it all together.