These days I find it pays to ask about the dress code ahead of time. When you set up the time for the interview, just ask whoever is setting it up how employees dress. If you ask this question in a straightforward, businesslike way, it should not raise any eyebrows:
You [to HR director]: “Jane thanks for coordinating this interview for me. I had a quick question about your company’s dress code: do the male employees generally [wear suits/wear business casual] in your office?”
HR director: “Well usually we are business casual,” or “well, your interview is on a Friday, and we are business casual on Friday.”
If you’re not comfortable asking the HR director or whoever set up the interview, you can also call the receptionist and just ask him or her what people generally wear on interviews.
Obviously if they are wearing suits, you should wear a suit. Note that a blazer and slacks is NOT a suit. A blazer and slacks is on the upper end of business casual, not the lower end of suits and ties.
If they are wearing business casual, but they sometimes wear suits (when they are meeting clients etc.) then you should definitely wear a suit, because for them a suit says “I’m taking this seriously.”
If they never wear suits, e.g. they are a tech company or something, then you should wear at the upper end of business casual. If you a wear a suit you are going to freak them out, so don’t do it.
Your goal is to be just a little more dressed up than they are. The obvious exception to this rule is if they wear suits: you don’t want to be more dressed up than that. Don’t for example, wear a tuxedo (for an evening interview) or a morning coat (for a daytime interview). (Ha Ha).
But like I said, the main rule is, if you’re not sure, ask someone there. First choice, friend on the inside. Second choice, HR person who is coordinating the interview. Third choice, receptionist or secretary who can tell you what people generally wear (and, if you’re nice about it, the receptionist will maybe even tell you what NOT to wear: “a guy came in here last week wearing a tie, and that was a BAD IDEA, because we are a very casual office.”)
And yes, get your shoes shined.
Constantine