You don't want slipping under normal operation. If you can do some acrobatics with your hands, hold one wheel and the spur, then use the other hand to turn the other wheel (front and back done separatly). There should be a lot of resistance, but a small amount of slip.
When you hold the spur and rotate a wheel, the other wheel at the end of the car should rotate easily, without a "gritty" feeling.
Agama A8 Evo | GRP .21 | Spektrum | Savox
My car just hit 88mph. I hope it comes back soon....