Polygon from overlapping Rectangles?

1. Overlapping rectangles -- HELP

I thought this would be trivial problem, but...

If I have the 4 vertices of two rectangles, is there a quick and easy way
to tell if they overlap.  I don't even need the overlapping region!

All I've thoght of so far is brute force checing several cases.  
eg.  one in other, other in one, one with vertices in other, other with
verticies in one, one with line through other, other with line through
one...  I must be missing the boat big time here!

Any help would be GREATLY appreciated!

Thanks,
-Gary

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