you can mark points using google earth.

You must make sure that there is no shift between your DEMs, which would yield false results. Ideally, they should be in the same projection and, if possible, have aligned grids. Then, digitize the points and use the "extract value to table" to get the elevation from both DEMs simultaneously. You can join this table aftwerwards to your points.

- Download Google Earth image of your area of responsibility

- Import and Georeference it in ArcGIS

- Navigate to the desired point on the georeferenced image

- And take readings of the selected points where your cursor is placed.

Create random points, and then extract the overlaying pixels values of the two rasters to that point file (extract multi values to points tool in ArcGIS). Export the attribute table of that point file into diserable format and use it for the RMSE calculation.

It's quite simple

Mark points on google earth

import on ArcMap or any over GIS software

Extract values for both DEM

Export data and do what you want to...............