There are two basic methods to see if the timing is correct. First method is the well-up method. A special capillary tube is screwed on the pipe connection for cylinder one of the diesel pump. Now slowly turn the engine over degree by degree (start at around 30 degrees before TDC in compression stroke of cylinder one), when you see the diesel rising in the tube, the pump is at injection. Now read the position of the engine. You can do it without the tube, but because the amount of diesel is so little, it is difficult to see without the tube.
The other method is with a drip tube. For this method you'll need to remove the spring and delivery valve. For that you'll need to remove the pipe connection for cylinder 1. Carefully take out the valve and spring, number 3 and 4 in the picture below.
Screw the pipe connection back on the pump (35 Nm maximum torque) and install the drip tube, the picture below is a drip tube:
Now you can pump with the hand pump, because when the valve and spring are still in place, the hand pump isn't powerful enough to overcome the pressure of the spring.
If fuel streams out of the drip tube, the pump isn't at injection, slowly turn the engine, degree by degree, until the fuel drips out of the tube, when the drips are one every second, you are at injection. Read the position of the engine. Adjust by swivelling the pump if necessary, you'll need to loosen the three nuts of the pump and the bracket at the back. To loosen the bottom nut, you'll might need to remove the left front wheel.
When ready install the valve and spring, and use a new washer, number 5 in the picture above (from MB or a Bosch diesel service station, not a generic washer) and perhaps you'll need to have a new rubber ring (nr. 2 in the picture above). First torque the pipe connection to 30 Nm, and than loosen, again torque to 30 Nm and loosen, than torque to 35 Nm. That is to let the washer seal properly.
Work clean, diesel injection systems don't like dirt.