Sadly, in North America, there is no rating beyond what each manufacturer decides for itself. I suspect that in the US especially, some thought goes into liability (i.e. if someone freezes to death in a bag that's rated to 0F and it's 20F out, they could be in trouble). Certain manufacturers gain a reputation for conservative ratings, others for optimistic ratings... and sadly, some manufacturers are all over the map (i.e. don't use an internally consistent rating system at all).
Fortunately, there is a standard that is (hopefully) coming to North America, called EN 13537. It is currently applied to all bags in the EU and certain manufacturers elsewhere.
The standard establishes a way to test sleeping bags, and then provides each bag with a set of ratings from an lower "you won't die" limit, all the way up to "you will be sweaty" upper limit.
There's another wrinkle though, which is that a lot of the warmth people attribute to their "awesome" sleeping bag, comes from a combination of other factors: sleeping pad R-value, sleeping clothing, tent heat retention, site selection, and what I'll call metabolism (i.e. whether you are a "cold" or "hot" sleeper).
So when you ask whether a 15F bag is really good to 15F, I'd say that it's impossible to know without getting the bag and trying it, at least in North America. I think if you know you have controlled all the other variables, and are a "hot" sleeper, with research you can likely find out which manufacturers are conservative raters, and go with them. In Europe, you'd just need to know where you fit in their spectrum of ratings and buy similar bags to that in a different temperature range (much easier!).
An example that might help explain what I'm talking about: