If your tank breaks and collapses, then you have too much flow. In all seriousness, it probably depends on what you are keeping. If your tank is 100% sps then you probably can't create a (realistic) situation in a tank that would be too much flow. You certainly shouldn't aim powerheads directly at sps. Obviously, if your tank is lps and/or softies, then yes, youcould have too much flow.
I believe you can never have too much, its really where you are creating flow and by what means. I know if its too much/wrong place to create flow if sand is blowing all over and my corals are closed up and tissue is being swept away. I have an mp10 on a 29 biocube and have that thing cranked almost all the way! Wiz.
This is what my 60g cube looks like when I turned both mp40's to 100%, just to see what would happen. Now I have to smooth the sandbed back out later...
I'm also curious as too how much flow is enough. Is there a video out there that can shed soome light on this. I randomize alot for my tank and whenever I get bored, I will play with the MP10s a bit.