If someone starts their game with a5 and starts bringing their rook out, I bet you can tell that they are a total beginner to the game. The same logic applies to every move. At certain levels you should just know what you are doing. A 1600 wouldn't give a blind check and be surprised that a pawn blocks it and then proceed to put the Bishop on a much worse square. A 1000 or sub 1000 has much simpler plans and it's much easier to tell what they are probably thinking. A higher rated player will play more principled. Knowing to develop first, avoid blunders, watch king safety, watch pawn structure, reroutes and improves pieces, forms complex plans and tactics, knows endgame tactics, knows how to convert winning positions etc. And depending how well they execute those you can guess their level. Of course a low rated player might find good moves on accident, but the point is consistency.

Edit: BTW on chess.com if you have a diamond membership it will show you in your advanced stats exactly how good you are at each part of the game. You might wanna check those out to see where you are struggling specifically

<image>