RE: PokéBeach's Official Global Link Codes and Events List | 5/9/13 Bianca-themd Pokedex skin! And B/W Kyurem C-Gear Skins!... soon, for a few people
There are other ways of determining legitimacy, or at least legality. I've been "serious" about trading for quite a while now and not once have I needed to limit myself to trading for Entree forest pokemon only. Sure, your method applies to male-only pokemon released only in certain areas like the DW Kanto/Hoenn starters, but readily available (and breed-able) pokemon (like the Amoonguss and Mamoswine lines) are usually bred to teach egg moves since most egg moves don't come on Entree forest pokemon (for example: Stun Spore for Amoonguss and Icicle Crash for Mamoswine), if the pokemon is even available on the DW itself (Foongus isn't, but it's perfectly legal with Regenerator, if acquired in a legal manner - not hacked in). Also, many 'newb' hackers just slap Entree forest on any pokemon they generate with a DW ability, often to get the moves they want on a pokemon they just caught the same species of in the DW, so your method isn't foolproof. If you learn what pokemon have been released, which are breed-able (have been released as female too) and what pokemon are region-specific (usually male-only events, like the Kanto starters), you can significantly lessen the possibility of getting hacked DW mons, and with a little further help with a legitimacy checker, you can ensure the pokemon is legal (and if not, you can call out the one who traded it to you) even if it doesn't come from Entree.