there are several options,
- run the code and use the base map of the Google Earth Engine to verify (easiest)
- find a classification with verified data then run the code and compare and contrast the accuracy of the classification method (more about the code less about the area of interest)
- buy a high res satellite image and verify the area (costliest)
- mark random or stratified sample point on the classified area and go to the actual location with the GPS coordinates and verify by your self (most accurate and hardest)
or
run following analysis separately
- NDWI for water
- NDVI for forests
- Time Series analysis for Crops and cropping patterns
- NDBI for built up area.
then verify the data from the code with the above analysis.