In general, while you can point to various characteristics that pubs usually have, and bars don’t, it’s always possible to come up with exceptions to the role. Overall, it’s often a case of “you know one when you see one”. I’ve suggested in the past that pubs are often specific buildings designed for the purpose, while bars tend to be part of a larger building. Pubs make use of the upper floors of the building, while a bar may be underneath something entirely different. The licensees of a pub are likely to live on the premises, but with a bar they hardly ever do. And, at least outside urban centres, pubs often have car parks, but I can’t think of a single bar that does. A pub retains its identity through various changes of ownership, while that of a bar is very much tied up with its current trading format.
Sometimes it’s less a question of physical aspects but how businesses choose to define themselves. On Stockport Market Place there are two recently-opened establishments right next door to each other – the Angel and Project 53. Both have a somewhat “crafty” ethos, but the Angel definitely comes across as a pub, whereas Project 53 is unquestionably a bar. With a new name and a different paint scheme, the Angel could be considered a bar, though.
Some Wetherspoon’s, particularly those in their more modern design idiom that are conversions of former retail units, do very much say “bar” rather than “pub”, whereas others than are in existing pub premises, such as the Gateway in East Didsbury, are definitely pubs. And, while their name says otherwise, I’d say that the vast majority of micropubs, going by the criteria set out above, are in reality small bars little different from the keg-only “box bars” often found in similar premises.
At the other end of the scale, there’s also the vexed question of when a pub actually turns into a restaurant. Most restaurants obviously aren’t pubs, but quite a few have the outward appearance of pubs and indeed might once have been one. Strictly speaking, if anyone can come in and have a drink without needing to buy a meal, it doesn’t qualify as just being a restaurant. However, I’d say there also needs to be a test of whether any meaningful number of people actually do.