If it’s posible to recommend a product earnestly,
then you must take for granted that it’s possible
to recommend a product dishonestly.
There’s no way to tell those two things apart when
done well.
This is true for some things, less true others.
For products that are almost purely subjective (books, movies, etc) yeah. There's really no way to spot well-concealed astroturfing.
As things become more objective the astroturfing gets a little bit difficult.
If I'm an astroturfer and I say, "I think brand XYZ leaf blowers are the best" but unless I go into some details that recommendation is not going to carry a lot of weight. And if I falsify objective details, things will start to smell more and more like astroturf to anybody that is actually knowledgeable.