A rule which works for me: The more obvious the suggestion in the name of the product, the less trustworthy the claims. Wacky spelling is also a giveaway.
So kind of like a scale from least:
Kwik-Grow Hair Serum bat guano amino nutrilium complex.
Eze-c***-Enlarge.
.
.
.Genhair
.
.
.
To most:
.Dutasteride/Avodart