Recognize pattern on a board?

Heya all, I'm trying to make up a board game where placing pieces in some pattern (like a square or a diamond) make stuff happen. But I'm not sure how I could do that in Unity...

I got my board tiles, I know how to all get them in a list, and from there I suppose I would check if tile are on them at the right places, but I'm not sure if there could be an easy way to do this... I mean, let's say something happen if I got four pawns in a square, anywhere on the board, then it leaves for tons of possibilities and checking for all of them would be long and annoying...

So would anyone maybe have an idea on a way to do it easier?

thanks in advance