A common giveaway for illegitimate sites is that Valve will block links and filter the name in the forum as ♥♥♥♥♥♥♥♥♥ (much like its profanity filter).
Steam users are being paid with discounts from these sites to add the site's name to their username, as this exploits a bug in the Steam Community where usernames are not filtered.
Most of these sites hide their servers behind Cloudflare, and use hidden WHOIS details, often via a Russian provider.
Here are a few sites I have seen spammed from Steam users recently:
List of domains/hostsskinhub.com
EDIT: Here is a statement from a Steam global moderator regarding gambling sites: