The example is Google Maps. Let's say I want to make certain that the code that I've written for placing markers on the map is correct. Is there an established way to test this? Ideally I would want to confirm that the expected marker(s) exist and are within a certain distance tolerance from the expected point?
This sounds like you are trying to test Google maps, rather than your own code.
If there was something wrong with the marker, is there anything you can do about it?
I would do some manual testing to make sure you are calling the Google API correctly and then leave it at that.
Are there some odd corner cases you are worried about?