If the truck is currently running an active regeneration cycle, you’ll know because the normal DPF capacity gauge will be replaced by a message that says “automatic exhaust system regeneration in process” That message will remain until the cycle is completed or canceled by the operator, at which point the DPF gauge will return and the message will disappear.
There is no way to know when the truck is currently regenerating passively as there is no message or indicator of any kind that tells you it’s doing that, passive regeneration just happens naturally as you drive. (Provided you’re working the truck hard enough)