Most of the time u don't need to know which transition to activate (unless in special cases of forks, decisions etc..)
will find the default leaving transition from the current node where the root token is parked and signal it to the next node.
If u really want to know which node the process-instance is in use this
Often my real-life boss is not happy when I say the source is the best documentation, but often, in combination with good testcases it is. jBPM has numerous testcases and you'll see that by looking at them, that they will answer many of your future questions