realistically u dont know unless u actually try to find out. the only way people end up really knowing is when they look into peoples alts or somehow figure out their other accounts and can see that they are queuing into themselves constantly.
usually really really high winrate (like 90%+) can be an indicator and gets people to look into that person, or the fact that 1 healer is always in the same lobby as the other healer or 1 of the dps, but again you dont really know unless you look. otherwise it can just be an inexperienced player that is slightly too high due to inflation or just someone having a bad game which is why its difficult to ban for this unless its someone wintrading themselves.