If your car has the Board Computer, You can tell when a regeneration starts and stops by observing your instant MPG readings.
The normal indications that a regen is taking place are your instant MPG falls considerably, your coasting MPG goes to around 70 instead of 999.9 and your idling Gals Per Hour goes to around 0.7 instead of 0.1 or 0.2.
Some people also get a smell of burning, but I never have. If you stop the car in the middle of a regen you may find the fan continues to run for some time. (Don't confuse this with compressor overrun from your Climate Control System).
If you don't have a Board Computer, I have no idea how you detect a regeneration in progress.
Edited by: Inchindown
Antara SE Nav 184 AWD Auto. Silver/Black