I discovered my game being sold on an unknown site
:-o
Would be interesting if and how they managed to actually sell it.
I just do not see the gain in asking for free stuff this way. Especially if they immedatly activate the game, they probably did not resell it. And let's be honest, you game is a few bucks, so how much work would it cost them to resell the steam key.
Unless I see the gain, I am more inclined to believe that they are building up their potential library and might still stream your game or decided against it, for whatever reason. It is early access after all. If your game has not gone viral and they have more prominent games yet to showcase, why do it now. If they reach out to you, they are probably struggling as well. A streamer that no one knows streams a game that no one knows. Somehow I doubt that this generates much followers.
But to topic, it is hard. There are some telltale signs that might indicate that the message is not individual as already pointed out. But even if, how could you verify that the streamer is, who he claims to be? There might be non streamers that just ask for free stuff.
Since you have a demo version, if that is presentable and has more than two minutes of streamable content, just ask your would be promoters to do some content with that. Not only will you know if they are who they claim they are, you would also know, that they are willing to actually stream your game.
Oh, and of course, if they are indeed a successful influencer, they would either just not ask you and just do it, or ask for permission. Not ask for free stuff.