The Signs and Symptoms of "Primary Clenching"
Migraine and tension-type headache patients typically
 present with some or all of these signs and symptoms
 
* Waking with a degree of discomfort/headache is “not unusual” or daily.
Although they may not wake up with a migraine, they rarely wake up "feeling fabulous".  On a 0-10 scale (10 is worst head pain imaginable), walking around with "a number" (anything other than a 0, that is, feeling fabulous) is rare *but unremarkable* by the patient.

* Unsatisfied / unresponsive   to typical preventive meds.
They have a history of failed preventive meds, or meds they could not tolerate due to side-effects.

* Unremarkable “typical” TMD, or history of failed tx
May have used an occlusal splint in the past (or currently), without significant relief.

* Toothaches &/or sensitivity
They flinch at the air/water syringe.  Their teeth are sensitive to cold (they'll use a straw for cold drinks).  They get "phantom" toothaches (xray inconclusive)

* Resistant to mandibular block
Their medial pteygoid has developed a pH that the manufacturer of the anesthetic has not anticipated it to be (due to the chronic fatigue of clenching against a clinically hidden interference).  After two or three block attempts, they'll say their lip "feels fuzzy".  Hours later, when they get home, they're numb.

* Require/desire daily meds.
Taking an OTC analgesic several times a day practically goes un-noticed by the patient

* History of dental trauma
For example, 32 year old female has two crowns and no other restorations.  No history of decay.  They crowns were placed after a cusp fractured off, years ago.  Anterior incisal edges have wear patterns.

* Chronic sinus condition and/or chronic stiff and sore neck/shoulders
Clenching in a protrusive position obligates an isometric contraction of the lateral pterygoids.  Since there is no medial strain, the TMJs are asymptomatic.  The coexisting distal pull/strain on the pterygoid plates of the sphenoid bone by the lateral and medial pterygoids causes a chronic irritation to the sinuses.  They may have been diagnosed as having "sinus disease" by an ENT.  The protrusive position of the mandible during the event obligates the trapezius for balance, thereby allowing for chronic neck/shoulder discomfort, or prolongs healing of neck/shoulder injuries.

* Unaware of / denies activity
Whatever you do, don't suggest that they are clenchers... you'll be accused of suggesting they're stressed-out people.