The word "dive" is in the joint's name
If a place is actually calling itself a dive by name, that's a surefire sign that some hipster who's never been in a real hole wanted to open a bar, but didn't want to invest in a vacuum or nice lights. Lots of ferns, though. There's always money for ferns.
The lighting's great
Most light should be provided by slightly broken neon signs, not something weird like an Edison bulb or, ugh, a window.
There's a great beer selection
If you order a microbrew, it should be a shot of beer to chase down some cheap whiskey. Otherwise, the taps should be "regular" and "light."