I've never been to this bar before, but it only takes a quick Google search to find the Yelp page complete with photos from the venue. It may seem silly, but reviews often have lots of valuable information in them and reading them gives you clues about what to expect. For example, a negative review may complain that all the patrons are wearing polo shirts and that it feels like a party at a country club. You now know that it is the perfect place to rock your favorite preppy look.
2. Don't buy something new
It is very tempting to head to your favorite shop and choose something that looks effortlessly amazing on the mannequin. That outfit may also look amazing on you, but that doesn't mean it is right for the event. One of the things I noticed while working retail was that my coworkers' thoughts on appropriate outfits for a dress code differed vastly from my own. Now, that isn't to say that I am always right or perfectly dressed. However, if you go into a store unsure of what is appropriate for your event and you seek help from someone with wildly different views, chances are that you will walk out of the store either completely disheartened or with an outfit you don't really feel comfortable in.
Instead of browsing outfit ideas at the store, check out inspiration on Pinterest and then take a slow stroll through your closet. Pull out all the pieces that seem like they might work without worrying about how they will combine to make an outfit. Try on anything that looks promising and see what you feel the best in. Refer back to your inspiration photos and use your staple pieces and accessories to turn a perfect piece into the perfect outfit. Also, don't forget to put thought into your undergarments so that you feel comfortable and confident in every piece you are wearing.
3. Bring a neutral layer
The best way to nail an unknown dress code is to hedge your bets. For example, a cardigan over a dress can take your look from dinner at a nice restaurant to brunch on the patio. A piece of fun statement jewelry stashed away in your purse can take that dress to party time with only a quick trip to the bathroom (or vice versa, you can remove a piece a jewelry that makes you feel overdressed). A neutral outer layer, such as a sweater, blazer, or light jacket, gives you the option to transform your ensemble. This weekend the party I'm attending is at a dive bar and although I want to look nice, I don't want to look nicer than everyone else there. I'm wearing dark denim and my favorite wedge booties with a taupe, sequined tank. Although the top is fairly laid back as far as sequined tops go (it is hard to see in the photo, but the sequins are clear), I am hedging my bets (and keeping my arms warm) by layering a black, moto-inspired jacket on top. When it is zipped up, only a couple inches of sequins are visible, so if I decide it is not a sequin crowd, I can easily adjust my look. Knowing that I have options with my outfit once I arrive at the event means that I don't have to fret over whether I am on point with the dress code before I go. (If this sequin and jacket outfit sounds familiar, it is because I am a huge fan of it and have talked about it here and here.)
4. Be confident
Your attitude affects how people will react to your outfit. If you are over- or under-dressed, take a deep breath, put on your best smile, and decide that it is not worth worrying about. Think about it this way: if the dress code were imperative to your enjoyment of the event--such as a restaurant requiring a tie and jacket or a venue requiring close-toed shoes--then the dress code would have been clear to you (especially if you did a couple minutes of research). So, make a quick trip to the bathroom to adjust your accessories and then let it go. At the end of the night, people will remember how funny and interesting you were, not that you were wearing a cocktail dress to a casual house party.