In a similar way as you would differentiate between an original paracetamol or a fake paracetamol (take any medicine/supplement for eg).

Could you?

No? Because who has time,money and knowledge to do so. We just go and purchase whatever the prescription doctor has written. Right!

It's similar over here. If he can bulk up using X so could I- at least that's what the advertisement says.

So the simple answer would be, NO you really can't differentiate between fake and original.

And high cost many times doesn't resembles originality.

You could try and see what works for you and what doesn't.

Supplements are necessary, but you need some knowledge on it.