If you click on their picture or name (from the messages, from your reservations list - shouldn't matter) it'll take you to their Profile page with their picture and verifications down the left, their bio (if they wrote one) on the right, and reviews just below that.
Just below their name it'll show how long they've been on Airbnb. If someone has had an account for years, but no reviews, it would be worth asking them if they've stayed with a host, and how that experience was. In some cases, they set up an account and didn't end up booking for a trip. Others may have stayed, but the host left no review. Rarely, but what to be cautious of, the guest was bad enough that the host canceled on them - until recent changes in the review system, the host wouldn't have been able to leave a review on why they needed to cancel the guest during their stay.