Or run through the BPMNModel manually to see what tasks are present in the model. This works good for future and past tasks, keeping in mind that any "gateway" logic you'll have to future out yourself, as the BPMNMOdel is an actual representation of the process-definition, and not how a certain process has gone through the steps. To find out what exact steps have been taken in the PAST, use the suggestion above by @caoyongviper.
