I think the most common instrument used for diagnosing insomnia is the DSM IV (now 5) Insomnia. The Insomnia Severity Index and the Athens Insomnia Scale have shown good diagnostic validity. Sleep diaries are very useful. But if you want to establish a diagnosis you need an interview and DSM, ISDC 2 give you clinical criteria.
Sleep diary and actimetry are the most usefull tools in the evaluation of insomnia.
ISI is a good tool for the diagnosis, but you must keep in mind that insomnia is a subjective evaluation of the patient : if the patient thinks that his sleep is insufficient or only "non restorative" there is an insomnia. Your job is to caracterise the insomnia and to propose an adapted treatment.
As Alain NIcolas said, sleep diary and actimetry includes objective measures in insomnia evaluation. 7-day actigraphy registers show consecutive sleep night mesurements and differenciates diurnal napping and nocturnal sleeping. It can be used in community dwelling persons, as institutionalized subject. And also is avalaible with laborable people with high diurnal activity. So you need to asess patients about accuracy and maintenance of 24 hr wrist actigraphy, because ocasionally could be unconfortable. Best wishes