I don't think that would every happen for a player, especially if you say every game, but even it did, there are a few points that would give others reason to believe. You can go into the game statistics and check CS scores, tower destruction, kills, deaths, who was involved in those kills/deaths, etc - basically everything that unlucky player would need to argue his/her point.
If he/she is consistently or at least frequently bringing in higher stats than the person he/she was laning against, it could be an indicator of skill held back by teammates' misplays. You might say that some people just play for stats - not the game - and yes, that is true, but a person playing for stats would likely have less signs of involvement in skirmishes and trades.
If the player has a higher creep score and is able to take out the enemy tower before his/her own tower is destroyed; that is a sign of lane dominance. If the player has quite a few deaths, you can still check the KDAs of the players on each side. If the enemy jungler has a high total assists to total kills ratio, it can be deduced that he took part in quite a few ganks, leading to the [X] amount of deaths the unlucky player had.
You can also check the locations of where the player died, if he/she was showing deaths in places that weren't near tower, you can probably assume he/she played too aggressively and suffered because of that. If the deaths were in areas protected by tower, there was likely nothing more that could have been done - especially if the deaths in those ranges had enemy jungler intervention.
There are a lot more signs you can take from just the graphs, but this is all I care to explain for.