Sometimes there are more obvious tell-tale signs of fake online reviews, but product manufacturers are getting more creative. As the New York Times points out, you sometimes need to dig a little deeper into the actual language to detect an impostor.
What words and phrases do you want to look for? High usage of the first-person singular, verbs, and adverbs tend to be indicators, but you'll also want to keep an eye out for specifics. References to the people the review was with (e.g. "my husband" or "my family") and the exact details of the product or service in the review (e.g. "The Superawesome Hotel in Downtown Chicago" or "My First Autopsy Playkit") occurred more often in fake reviews than real ones. In fact, these indicators tended to be the strongest (along with usage of the first-person singular). While you can never really know for sure, if you're concerned about trusting a review you might want to look for these criteria.
Hit up the Times for a look at the full infographic, and check out this study for a little further reading.
Is That Review Fake? [New York Times]