I'm not entirely sure how StockPromoters.com works, but one way or another, the promoters report their compensation. A kind of honor system, and there's not a lot of honor amount touts.
I think some of them send the info to StockPromoters; StockPromoters may rely on website disclosure pages and email alerts for information about others. I know that most of the APS companies didn't report for a long time; now information for them is available.
That said, it's generally impossible to figure out how much they're really being paid.
The people doing "free" promos are generally frontloaders. Probably they accept compensation when it's offered, but when it isn't...