I am with Raster Calculator in ModelBuilder on ArcGIS Desktop 10.2.
In the Raster Calculator's Map Algebra expression window, I need to check if a Raster dataset has already existed or not.
Can IsNull do this job?
No, IsNull will not check if your raster exists, it tests for NoData (null values) in an existing raster.
No, you can't check if a raster exists in the Raster Calculator.
You need a precondition in your model before the Raster Calculator.
Using If-Then-Else logic for branching (arcgis.com)
If Then Else in ArcGIS model builder (gis.stackexchange.com)