I imagine you would have to set up a profiler trace to monitor if anyone is accessing the cube. I'm not sure about any other logging build into SSAS but someone else might know.

Is the cube still being processed? You could use the query below to check. If it hasn't been processed in a while then you could always stop the service and wait and see if anyone shouts

SELECT CUBE_NAME, LAST_SCHEMA_UPDATE, LAST_DATA_UPDATE FROM $system.mdschema_cubes