You use a pressure biofeedback device like this one:
http://www.amazon.com/gp/product/B01...f_rd_i=desktop

No need to buy the expensive gadget. One can simply take apart the old blood pressure cuff (the old-style with the pump and dial, not digital).

Here is the exercise (longus colli strengthening):

1. Lie on your back with knees bent
2. Place the pressure cuff under your neck and attach the meter where you can see it without moving your head. Inflate to 20 mmHg
3. Tuck the chin slightly to increase pressure to 22 mmHg
Make sure not to use scalenes or SCM muscles (feel them with your hand).
Hold for 5 seconds, relax for 5 seconds. Do 30 reps twice a day.
4. If this is easy, increase to a 10 second hold and then progress
to 30 or 40 mmHg.

This exercise is not hard to do. The idea is to learn to engage the right muscle for neck flexion and relax the scalenes.
I am not sure how fast one should progress and what target pressure is the best. I was adding 2mm pressure every few days. After a while I stopped using the meter, just inflated to the pressure that felt right.

I did this exercise for 3-4 months and found it helpful. Don't think my longus colli was actually weak. I was just using a bunch of other muscles along with it.