The "garbage-collection statistics" RTS options can be useful for seeing what parallel programs are doing. If you do either `+RTS -Sstderr' or `+RTS -sstderr', then you'll get mutator, garbage-collection, etc., times on standard error. The standard error of all PE's other than the `main thread' appears in `/tmp/pvml.nnn', courtesy of PVM.
Whether doing `+RTS -Sstderr' or not, a handy way to watch what's happening overall is: `tail -f /tmp/pvml.nnn'.