You can ask around on #littlecup, but you should be safe as long as you follow a few simple rules.

If a Pokemon is genderless or male-only, it can't have any level-up moves it couldn't get before level 5. This is because they can only breed with Ditto, so they can't pass down level-up moves via breeding.

However, in gen 3, it was possible to do so using the Pomeg Glitch. That's why stuff like Hydro Pump Staryu and Tri Attack Porygon are legal (because they could get them in gen 3)... they just can't those moves with abilities they couldn't get in gen 3.

Also, certain Pokemon are released from the Dream World, however they can't be gotten at level 5 as they were released as male-only or genderless (so they can't breed the DW ability). So that includes stuff like Analytic Magnemite/Staryu/Porygon, the Kanto starters' Dream World abilities, Heavy Metal Bronzor, etc.

There are a couple of other minor moveset illegalities too. Pokemon can't use Gen 5 exclusive level-up moves/egg moves/abilities with Gen 3/4 exclusive tutor moves. I can't think of any examples of this off the top of my head. Generally you'll have to check for yourself with this (though tbh nobody would notice most likely, and if anyone does it's not like you'll get banned on sight).

Don't stress out about this too much though. If someone points out something is illegal to you, just change it and you'll be fine!

Sorry to be a pest but how can I find out if my Little Cup team has an illegal combination? I don't think it does but it's better to be safe than sorry. : O