It's ok to ask anything.
Of course, asking this sort of question may feel awkward. And being awkward while asking a reasonable question doesn't always make a good first impression. You may want to ask someone other than the hiring manager, if you can find someone accessible (if you did a phone screen with a recruiter, for example, that would be one good source for asking a dress code question).
And if the company is local, you could swing by a day before the interview and check out the place from outside. As you see people walking in or out, you can get a sense of what they are wearing.
If you still can't get a good sense of appropriate interview wear, then dress up and wear a suit. Overdressing is usually considered less harmful than under-dressing.