Generally by volume. One or two? Sure, you might be a really generous friend. Fifty? Probably a bit too generous - something fishy is going on. Of course, without a federal firearms registry, its difficult to quickly just flag people who are buying guns at an unusual rate.

As far as actually catching perpetrators? Again, without a database its usually down to undercover work and tips.

https://giffords.org/lawcenter/gun-laws/policy-areas/crime-g...

https://giffords.org/lawcenter/state-laws/trafficking-straw-...

https://www.fox32chicago.com/news/56-arrests-dozens-of-narco...