Lichess doesn't determine the estimate ratings of the players, only chess.com does. Do note that you can only know the game performance of each player, not their actual rating, but usually it wouldn't be that far off.
Expanding on @qwr's comment, asking a strong player would be a good idea too, because they can tell how well the players are doing. But again, you can never find the real rating of these players, for example one of them could be really sleepy and throw the game.