There must be a clearly identifiable disclaimer attached to the testimonial itself. If a company has paid a celebrity influencer for a promotional testimonial for a social media post, they may choose to use a hashtag like "ad" or "advertisement" to satisfy FTC guidelines; In short, the relationship must be disclosed in some publicly identifiable way.
DON'T copy & paste reviews from other sites. When a review is originally posted on another review site, it's inherently under the ownership of that website. Most review sites relay this in their terms of service. Failing to comply with such terms results in the infringement of rights.