I performed IDW using ArcGIS on 121 evenly distributed samples and generated a surface.
How do I check the accuracy of the method?
In order to evaluate the accuracy of your interpolated surface you either need:
These are used to cross-validate your result.
On your created IDW layer you right-click and choose Validation/Prediction.
This link actually describes the process: https://desktop.arcgis.com/de/arcmap/latest/extensions/geostatistical-analyst/using-validation-to-assess-models.htm