If a have a huge project and I finished assigning ressources to it, how can I see or be notified that a task in the whole group his only partially staffed, let's say at 50%?

Found it! So, for the record, Resource>View>Resource Usage and look at the line Unassigned

Also, another question I had and found a way: How can I most efficiently see where I have bandwidth?

Answer: Resource>View>Resource Usage, right click on table view, select Remaining availability instead of work and bingo!

Thanks for posting the resolution, Patrick.