Exactly! You can't know, although I would say that you can have a reasonably good idea most of the time, by just going by set social standards and community rules-of-thumb. Like I mentioned above, for example, greeting places are frequently more lax about PDAs than, say, restaurants or middle-of-the-sidewalk.

Basically, my complicated-"it depends"-answer-that-would-never-be-an-option-in-rosa's-polls is: it's inappropriate if other reasonable people are made uncomfortable, but since you can't put a logical and clearly set definition on "reasonable" or "people" or "uncomfortable," you have to use social senses to choose which side of the "possibly uncomfortable" fence to err on based on surroundings, and I, personally, tend to err slightly on the "inappropriate" side.