It's always best to dress nice--button down shirt and tie for men, skirt/dress for women--and then check out how everyone else dresses. If you come into a casual shul well-dressed, if your clothes are even noticed, people will generally think well of you. If you come into a more formal shul casually dressed, it will make a bad first impression. It's kind of like an interview that way: always better to dress nice to start with, then casual down if allowable.
Also, many Friday night services tend to be more casual, because people go after they get off work. So keep that in mind if you see everyone wearing jeans and casual clothes on Friday; they may wear ties and dresses on Saturday.
I normally don't contact anyone ahead of time, but if you're only going to do one service, you might ask the office which is better for a newcomer. Some synagogues do not have many people on a Friday night, while some do not have much of a Saturday service. And some might have a casual, instrumental Friday service and a formal Saturday service, or one's more family-orientated and the other more traditional. But if you have the time, go to all the available services at a Shul to get the overall flavor of the community and see which service best suits your interests.
And if you don't like calling (I am shy and don't like to call if I can help it), the vast majority of synagogues have websites and they not only list their service times there, they will have an e-mail address.