Your question is in fact two-folded: 1. If someone/something has free will, what behavior will he display in contrast to someone/something that has no free will. 2. Does Data show this behavior that implies free will?
Currently it's heavy debated if humans have free will and hence it's heavily debated if any behavior we show is one that indicates free will. So question #1 can't be answered easily. So let's assume, that humans have free will. Now we can compare Datas behavior to ours?
His decisions can be predicted. So can ours.
Sometimes he will do what he is told (e.g. scan for lifesigns) and sometimes he will refuse to (e.g. have his brain dismantled). So do we.
This decision is based on reasons (not to be confused with causes). With us it's the same.
There are some things he will never decide to do (e.g. if he's not messed with, he will never decide to hurt Gordie) - due to his programming. So won't we - due to our education.
Also due to his programming he has a set of things he tries to avoid (e.g. death of comrads or himself) and a set of things he is trying to achieve (e.g. become human). We are exactly the same.
So I'd propose (avoiding the first question) that Data has a free will, if you think we have a free will, and that he does not if you think we don't have on either. Now it's up to you: Do we or don't we? Philosophy-stackexchange is one click away!
