I know that this may be off topic but I would like to spend two more words on this
Just wondering how google would know the difference.
Try to move your focus away on humanizing Google. Google doesn't mind if you've REALLY paid or not for a content, don't think that if someone stops you in the street and ask you for a link for 5$ google may be aware of that, because it doesn't and I think that it also doesn't care about the REAL truth.
Google only cares about what IT thinks. Even if your link is natural and free, if it comes from a link network which sold links but in your case not, then you've paid that link too. Google is a private entity, which plays on its own rules, not bad nor good but just its own rules
Bear in mind that Google is not human, it's not under the human law of being truth when evaluating links, condemning the paid ones after demonstrating that they've been paid showing an invoice as a proof.
So when you analyze your backlinks (or trying to achieve new ones), try to understand if those links may be paid or not under google eyes not yours.
About being paid for a review you can create good content even being paid, just say to the one who's paying you that you will be writing a review but not publishing their content. You'll use their services and then write a review, so you're compelling and writing soemthing really useful for both users and google.