Primer, lots of light, and magnification.
The neutrality of grey primer makes it particularly well suited for inspection of flaws and previous repairs, easier to read than white or black. It will show everything you need to see. Apply primer and examine thoroughly (with light and magnification), noting imperfections. Then lightly sand with a fine abrasive (preferably block sanding), clean, and inspect again. Address all issues and repeat.
For inspection of heavy body mods, especially those that required a lot of filler, use the guide-coat method. This requires two colours or shades of primer. Apply one colour, once dry apply a light coat of the second colour. Then block sand the area, low spots/high spots will be revealed in the contrasting colours/shades.