Some teams had 5 players with their team name in front, others had 5 of the same #, and I would just ask in the chat after the game if they were solo or premade. This is of course not 100% accurate and people could lie if they wanted to. 9/10 times when you have a 4 man stack on your side they will have a 5 man stack, the last 2 games I played I had 4 premade on my side, where they had 5. I would say this occurs a lot more to me after hitting high dia/immortal, I guess most people just spam games together in that rank. Also, Overwatch and Valorant are 2 different ranking systems, Riot Games have the formula to add Solo/Duo/Trio and flex queue, they just need to read enough complaints about it, because no one wants to play with 4 man stacks, and no one wants to play against 5 man stacks if they are solo queue.