Without knowing any details about the datasource (and what's coming from it) it's hard to make a specific suggestion.
Assuming your datasource was going to have a data field that had a value of 1 for runner on base and a value of 0 (or anything not 1) for a runner not on base you could put something in visual logic like this:
This wouldn't work if your data source logic field was using a word instead of numbers though.
#XPression