Apart from the behavioral differences explained above, color is also a good hint as to what you're dealing with.(taking into consideration that it can sometimes be difficult to distinguish details when in flight) For the most part, your local hawks are going to be some variation of white, red, and brown (though Red-taileds can appear in dark morph), while vultures are primarily solid black. Zone-tailed and Black Hawks, both of which are quite dark, reside in the Western US. I'll try to find flight pictures out of my million bird pictures when I get home from work.
~Alex
I used to have a book that showed the bird outlines from underneath, which was helpful, but my eyesight is not that good anymore so after seeing vultures daily for 13 years I know them well. Hawks were a rare sight so maybe a lot of vultures ruin the neighborhood for hawks. I wish I had taken pictures before I moved of the trees filled to the brim with vultures.