I made a little Python script that takes a gcode file from Estlcam (or maybe other sources – but currently not that many gcode features are supported, so YMMV) and generates an OpenSCAD file which simulates the cut with an adjustable bit shape (you can adjust the bit tip angle, the diameter and the length). The cut gets subtracted from a solid representing the workpiece (by default a 1000x1000x6mm “cube”, but can be adjusted).
This should work better for some purposes than Repetier Host or Estlcam preview.
python gcode2scad.py filename.gcode > filename.scad
OpenSCAD rendering is slow because of the subtraction, and Render may work better than Preview. (You can also not subtract by setting workWidth=0, which will increase speed.)