well, not all who charge for their channeling is a fake, just 95% of them. there are some tells, for example leading questions, like with Dorothy and Professor Marvel [the Wizard] when he was gazing into the crystal ball. he claims he sees a woman who cared for Dorothy in sickness, at which point Dorothy opened up about the time she had the measles and her Aunt took care of her. many fake psychics do this and most people don't pick up on it.
if the person was suddenly possessed by the spirit you were channeling, i would also raise a flag. i have dealt with spirits, and have many friends who can speak with spirits as well, and never has anyone of us had a spirit enter our body and speak through us. normally a voice or strong feeling will come over one or all of us and we speak for the spirit.
while i'm not a fan [my mom is] the Long Island Medium would be a typical experience. she claims a strong feeling comes over her, and 'spirit' tells her what to say, or she will see a red car, or smell lilacs, and while it means nothing to her, it's the spirits way of relaying a message to the living whom she's speaking with. [i'm not saying she's a real medium or a fake, it's an example]
take everything with a grain of salt, and if your friends friend asks for payment, say no and walk away.