Chris,
You could always keep track of the length of the Annotation List array and if it increases, then a new annotation has been made. You can get the annotation list with a property node, so it would be relatively easy to keep track of. There is a good example of adding the annotations programatically and it uses this annotation list array. It is found in the LabVIEW examples if you search for annotation and is called "Programmatically Annotate A Graph".
This annotation list is an array of clusters where each cluster is a single annotation. You can change the color of the annotation programmatically as well by manipulating this annotation list property of the graph. I hope this helps.
Brian Coalson
Software Engineer
National Instruments