So you've got the metal smooth to the touch with use of a grinder or maybe a DA but it's nearly impossible to tell how straight metal is in bare form. The quickest way to see where the high and low spots are is to shoot it with some primer and sand over it with a long block 30degrees diagonal from the body line. The length of the block keeps it from conforming to imperfections on the panel and will remove the paint from the high/level areas. If there is a low spot that is too small to be pushed up with a hammer and dolly, you can fix it by applying a little bit of body filler.
A lot of people abuse filler, and put it on everything without trying to fix the sheet metal's shape. First mix the filler as the directions describe and spread over the low spot. Keep in mind most of this will be sanded off so don't put so much on that you are making extra work for yourself.
When the filler is hard, give it a rough sanding to get it close then put a contrasting colored spray paint mist over it. This is called a guide coat. when all of the spray paint is gone, your panel is flat.
Assuming this is the only part that needed to be worked on, you're ready for primer and paint!