In this, as in any other situation where you're unsure of the dress code, I think the best option is to split the difference and dress up a bit the first time you go.
No one will be offended, nor will you stand out oddly, if you show up in a day dress, nice skirt or dressy trousers-- think the business end of 'business casual.' Then, once you have an idea of what everyone else wears, you can adjust accordingly.
Also, if you're looking for a more casual environment, make sure you go to the family service (if there is one). These are usually the later (10:30/11:30) services, and tend to attract families and younger people. The older, more conservative members tend to gravitate toward the early services, in my experience.
Another thing you could do is see if the church has a website. Most do these days, and they'll often have a newsletter and photos posted online. You can get a 'sneak peak' at the congregation, and also read through and get a feel for the church and minister's philosophy. Do they use words like 'welcoming' or 'family' a lot? Do you use 'traditional' or 'orthodox?' I just always like to have an idea what I'm walking into.