Sometimes it's not very obvious if that hole-in-the-wall you've stumbled upon is even open. Sometimes it's not even obvious it's actually a bar. Generally, if there's a light illuminated the sign, entrance, etc. then yes they're open. If you still can't figure it out, just try to open the door (don't bother knocking), assuming there isn't some requirement to use a doorbell or intercom. If the door is locked, you have your answer. If you go inside and there's no staff and the place looks closed, you still have your answer. If it's late and you're not sure how much longer the bar will remain open, just ask the staff what time they're open till. Most likely you'll get an answer what time their last order is, and most places will remain open 30-60 minutes after that.
I’m Whiskey Richard, and I am the founder of nomunication.jp. I’ve lived in Tokyo for over a decade, and I am a certified Shochu Kikisake-shi/Shochu Sommelier (焼酎唎酒師), Cocktail Professor (カクテル検定１級), and I hold Whisky Kentei Level 3 (ウイスキー検定３級). Click here for more details about me and this site. Kampai!