As of time of writing the top-voted answer basically says you don't verify that the opponent can cast a Companion card, until proven otherwise. So for example if opponent is playing Keruga, the Macrosage, then they can't play any 2-drops. They could conceivably draw them, but since they can't play them, there's no unfair advantage gained.
Let's say instead I'm playing a Fires of Invention deck, which is heavily reliant on drawing the Fires of Invention. So I play eight copies of it. That's illegal, but it doubles my chances of drawing the card. How is my opponent going to verify I'm cheating now? After all, they need to see five copies of Fires of Invention before they can verify that I'm cheating, and that's not very likely in a real game. Otherwise:
- I can decline to play the fifth copy if I draw it (this works because I don't usually need to have more than one Fires of Invention in play at a time).
- If opponent casts Thoughtseize, I can concede before revealing my hand. Same goes for opponent casting cards like Peek, Surgical Extraction, etc.
- If opponent casts something like Glimpse the Unthinkable, I can put the cards in the graveyard one at a time, and concede if the fourth Fires of Invention is milled (thereby ensuring that the 5th-8th copies will never be milled).
I'm pretty sure I'm gaining a legitimate advantage by doing this, but since one can never prove that I'm cheating, one can't call a judge either. How is the opponent supposed to verify that I'm not cheating then?