Certain types of surface marring (specifically, rotary-holograms) will *only* show up in natural sunlight. If I can't pull it outside/into the sun to check, I might even assume it has them and I use the PC/Cyclo with the appropriate product to remove 'em.
But any other marring (including very light swirls) can be seen under the right artificial light. Halogens work well for many people, but for me, the most unforgiving light is high-wattage incandescent. A regular "old fashioned" trouble-light can work fine- just move it around while inspecting the finish from various angles. There'll be one combination of light position and viewing angle that'll make every little flaw visible. WIth halogens, incandescents, or anything else, it's finding that one combination that makes the difference. Just pointing a light at the vehicle usually won't reveal much.
Another trick is to use a flashlight in an otherwise darkened garage.