Most 3rd party sites have a review function.  If say you book  a  Hotel on lastminute.com once you have departed the hotel you will be asked to review your stay. 

The good thing about a site such as lastminute is you can only review if you have stayed at the Hotel you have booked.

Tripadvisor – Ok but keep in mind the following.

Anyone can post a review once registered (you can pretend you stayed in the hotel)

Hotels can post good reviews of their Hotels.  Competitors can post bad reviews on another hotel.

Disgruntled ex staff can post a bad review. 

I once worked for a Hotel in waterford and we were told a former staff member had posted on tripadvisor.  They said they had got food poisoning at the hotel.  I asked tripadvsior to take it off.   They said they would not as they encouraged free expression of views.  I asked can I post reply criticing  Tripadvisor for this.  They said no. Free expressions can be a one way street.

I do believe that most of the reviews on Tripadvisor are honest & helpful but buyer beware.

Hotel Review Sites. The inside track is a post from: hotelconsult.ie