It’s not always a completely accurate equivalent, but as a way of getting an initial grip on what would be more natural in any given situation, you can also try an “as for (the)~” replacement with は. If you could do that as even a remote possibility in English, it might make more sense to use は. (You’re shifting subjects, but it’s something both speakers will be aware of as being within the sphere of conversation–or it’s otherwise not unexpected to bring up.) Basically you’re emphasizing the subject/topic.
But if you couldn’t, が is probably the right choice as just a straight-up grammatical subject marker. (You’re emphasizing, for example, that something is coming, more so than the fact that it’s a/the bus in particular.)
So yeah, in the example above, if you’re both travel-planning, or have already been talking about when a bus will come (like, maybe you’re waiting at a stop and one of you is looking up time-table information), は could make total sense. “As for the bus, it comes here at 5:15.” “The bus (which we both knew would be a part of our travel-planning) stops there.”
But if you’re just somewhere and want to be like, “Hey, a bus is coming,” that’s a situation that fails the “as for~” check pretty handily, so use が.
This is more helpful in eliminating は as an option than it is in determining when you should definitely use it over が, but it’s something.
By the way, this is why you’ll never see は paired with question words or ambiguous subjects like だれか. You’d never be like, “As for who, are they coming?” or “As for someone, they’re coming.” It would be extremely, extremely bizarre to emphasize the subject when there isn’t a clear one to begin with.