I have really bright kitchen LED lights which I use for this sort of test. To be honest you have to be prepared for things to look much worse than you will ever see under other circumstances, and likely issues that will never cause a problem. Case in point I have the simple Leica 50 Elmarit M, doing this test I get 2 or 3 faint lines which I determined must be fine scratches inside the lens, it drove me crazy trying to clean the outer surfaces and re-inspect but those surfaces look mint, moving the lens around under a light indicates those marks must be inside the lens. I reckon someone took her apart in the past and put those fine scratches in there 'servicing' the lens. I have taken some of my favourite photographs with that lens and scanned at a real 5400 dpi it shows the best results I have ever got from a film camera along side my other Leica lens. Its an amazing little lens, I can't seem to get a hint of anything amiss into a photograph. The typical case for a flash light test is for it to light up any and all tiny bits of dust in a lens, also any kind of oil or smearing will show up as a faint ghosting. I have bought quite a few secondhand lenses and typically the rear element lights up like a snow storm as most people never clean it. You have to decide if its worthwhile bothering cleaning a lens surface if it looks mint under reflected light and only looks dirty/dusty under a flash light through the lens test. Personally I wouldn't bother unless it was really bad.
By the way I did this test to a fair level of attention on an SWC I recently bought following the Ken Rockwell note about the old Compur shutter scratching the inside elements in the lens. Wow does that lens look clean inside, its a lovely remarkable thing to see a 40 year old lens which optically looks really close to mint. This is what I love about the secondhand market. People rate and price based largely IMHE on physical cosmetic condition which is irrelevant, so you can find something marked exc because of cosmetic wear but with mint optics. Also some shops don't seem to bother cleaning lenses so you can get some real bargains where the lens has been marked down but only needs a gentle clean to return to near mint (why on earth would folks trade in a dirty lens?).
Its always difficult to predict what will show up in a photograph, dust almost certainly doesn't, ever IMHE. I have a big piece of dark dust inside a lens which doesn't (as yet!) show up. Contrary to what some say though I did have a case several years ago where an oil spot on the outer lens element was showing up in many photographs (digital) as a small smeared area, therefore I clean those off unless its right near the edge.
