How are positions visually indicated in the UCSC Genome Browser? I'm a little confused about how the position indices work in the browser. Based on the picture below, is base number 755440 an A or a G?

The general question is how do the indexing guides work? Is the base right under the | the one corresponding to the index or is it the one just before the |?
It's an A. The number refers to the position just before the |. This is clearer on different zoom levels, but I double checked by going to the very beginning of a chromosome: 



As you can see in the image above, the | is always after the base it refers to. I also confirmed by checking a few random positions using another genome browser which places the positions unambiguously above each base. 

For more details, see this UCSC blog post.