yeah, A unique (downloaded/pack girl) will usually have a blurb in the girl management screen, and not just a 'type' eg cute girl and no blurb. Also, in the slave market, non-random girls are in red. You'll identify the girls from their pics ofc, but if your looking for a way to know from their names alone, it's from your memory only. Uniques have names set for them, randoms have japanese names. So you know Cammy White is unique, Rika is random.
If you can find it around here, there's a file that widens names to non-japanese so randoms can be called veronica ect. Might help if you have many names in japanese that only have the last part different.
I think walking can give you randoms, it depends on how many girls you've downloaded. It'll use up all the unique girls first though that aren't set to 'catacombs only'. Oh, yeah sometimes you cannot get all your uniques UNLESS you send gangs/girls into the catacombs.