Star Trek: 10 Things You Didn't Know About Data
1. The Question Of His Sentience
In The Measure Of A Man, Data fought a legal battle to be granted the same rights and freedoms as organic Starfleet officers. In the end, it was deemed that he wasn't the property of Starfleet, but his status of sentience was still up in the air.
Data could not be read by most telepathic species, and some believed he was no more self-aware than a simple computer. Over time, many people would come to respect his individuality as a new form of life.
When B'Elanna Torres mentioned Data in Voyager, she said that he was treated the same as any Human, and described him as the only sentient artificial lifeform in the Federation (despite working with another sentient artificial lifeform aboard Voyager). This seems to suggest that Data's sentience was well-known and generally accepted.
Although Data may have been deemed sentient, other self-aware artificial lifeforms like Voyager's Doctor had to continue to fight for their freedom. We've still yet to see the rules that determine when an A.I. can be considered alive, or if any such rules even exist.