Hola, Good question! We, the Japanese people also have a tot of kanji that we don't know, especially those for people's name and the name of the places. So asking how to read is very normal for us. Besides, there are so many kanjis which are read irregularly in their names though they are very simple kanji or they can be read in several ways. So we can ask how to read them correctly. Espero que mi inglés sea entendible y que te ayude. Si todavía tienes duda, me dices. Un saludo.

I think, you can just ask people about it. Since it's written in a public place, somebody would know the pronunciation. It's even easier at a restaurant. Like, "Oh, I'll order this, that, and... I'm sorry, how's that thing called? I can't understand this kanji". I don't think people would ignore you. You also can search it up in a dictionary (almost everybody has a smartphone now). Or, you can just say something like "this building/thing" (without saying it's name), and search for the unknown word when you'll get home.

Hi there :D Nice question ^_^ What do you do f you are at a restaurant and you see an unknown kanji? You look it up in a dictionary(even if you don't know how to say it, you can just draw the kanji on the display and the meaning will pop up). Same thing on street, look it up :D Please ask if you have anymore questions!

Not everyone does actually have a smartphone or a dictionary available, so any other ideas?