I've done quite a bit of radar work at club fun-fly events, and have found it pretty easy to do, but that's using a $2,000 radar unit.
Bushnell and others sell lower cost "speed-guns" for use with baseball and other sports. These should give acceptable results with R/C planes if you remember a few basic principles.

Unless you're using a laser type, the radar signals goes out in a cone shape so there’s no need for a viewfinder, just point and shoot in the general direction of the plane.

Limit the angle between the radar unit and the passing airplane as much as SAFELY possible. The greater the angle, the less accurate and lower the displayed speed will be.

The radar signal has to be reflected back to the radar unit in order to obtain a measurement. The spinning prop is the biggest target, but like most of our materials it's non-metallic so the entire plane is somewhat stealthy. The bigger it is and the more mass it has, the easier it will be to pick up.

Watch your background. If the radar "sees' something bigger behind or in front of the intended target, you'll see interference and have to interpret the signal in order to tell what's going on. Remember the cone shape: even if it's not in a direct line, the radar unit might pick up on a stronger reflected signal, bounced back by a larger object in the area.

I’ve successfully obtained radar results on everything from a 14mph all wood glider, to a 220mph turbine powered model and it's always interesting to see how inaccurate our own visual/mental estimations can be.