As the sibling says, it is easy to think about in terms of what you are doing with the data. Reporting on how many transactions of a type have happened over a duration? Probably OLAP. Really, if the word "report" shows up, at all, probably OLAP. Executing parts of a workflow for an active transaction? OLTP.

Briefly looking, I can't find the books that I thought covered a lot of this in a good way. Will keep looking, apologies.