I think the buildpower resource-bar should be enabled by default. It shows how much % of buildpower is being used. If I have 100bp, all of them assist a unit, but I only have the income to support 10bp, then the bar is only 10% full and 90% empty.
By the way, I keep saying this, but if anybody didn't hear it yet:
When spectating a big teamgame, let the game run for a few minutes then press CTRL+A to see the total used/available buildpower of all players. You might notice that it's always somewhere around 2/3. I just think that's funny and really interesting.