It depends very much on the person and his/her relationship to God! I can speak of what I've gone through...
The first impression is of course of the way every meeting is organised (prayer, worship, sharing, even seriousness during the meeting) etc. I think we tend often to look for a church that more or less looks like the church we have visited before. (I hope, I've outgrown that now). But that's the first thing.
Secondly, the maturity of the elders and the doctrine. Usually when I visited a church for a first time (for alas, I have visited many churches in my searching), I spoke with the pastor or the leader. I want to understand usually what's the basic doctrine of the church. But I know that there may be people who may have the same Christian manifesto but very different understanding on "What should the normal Christian life look like?" or what is the grace of God. So just because we agree on doctrine, doesn't mean we have the same mind.
Personally I think that the assessment of a local church for me comes after visiting 2-3 meetings. That may be "pretty fast" for some, but personally I think that by this time I have tasted enough of the spiritual atmosphere and could make rough assessment without knowing all details. For example at one church, later came up issues that clearly showed me that "something" was wrong and I left.
The other thing that's pretty important is fellowship. And I hope you really understand what I mean. Not some minutes just speaking about God, but believers sharing in small groups even after the service their burdens and their joys, their hearts glorifying God spontaneously! It is so wonderful! And it is sometimes so rare...! We should always renew our mind, so that we don't turn into a church with much traditions and no life...
One last note, those that come and are really born again believers - take time to know them and to speak with them. Or even some of the brethren in the church. Don't take people in the church as objects or visitors. Cause there is often a hungry soul, desiring to be fed; or a broken heart that dares not reveal itself... All people should be welcomed (but I speak now particularly of believers).
I came with a broken heart in a number of churches... and sometimes I could not speak with the pastor because I had a feeling that he will not understand at all what I was talking about. Broken heart comes often after broken relationship (between the church and an individual). Because, being part of a church is a commitment, not hobby...