1985 300TD, how to check if turbo is kicking in

The turbo is probably working as it should be. However, its boost pressure may not be getting to the sensor inside the injection pump. That's usually because the "banjo fitting" (on the very aft end of the intake manifold) is clogged. Simply remove the (hollow) bolt and use a piece of wire to clean it out, also clear the fitting on the plastic tube. Then follow that plastic tube to the injection pump to make sure it's not broken or disconnected.

The turbo's maximum boost pressure is about 11 psi. A pressure gauge attached to the banjo fitting can confirm that if the problem continues.