Testing this is likely to cause them considerable discomfort later on. That is for most clients as most have weak deep neck flexors. I tend to test at the same time as giving a home exercise.
Get the client to ly flat in supine. Then ask them (with their mouth remaining softly closed) to take their chin to their sternal notch. I get them to hold this position (without protraction) for 3-5 seconds and release. They might do this 3 times only as a daily exercise during the beginning of a neck retraining program. I warn them that they are likely to feel stiff initially and might also feel a sensation as if the tongue is being pulled back into the throat when performing this exercise.
I suppose I am not putting a number on the actual strength but does one need to? 8o