Lots of validated scales for depression. Make sure that the scale you choose matches the population you are working with. For general populations you can use something like the CES-D; for clinical populations, in general hospital or primary care outpatient settings you can try the GHQ or the PHQ9. For clinical populations, where there are likely to be higher prevelance and intensities of depression, use a specific instrument such as the Beck or Hamilton depression scales. for medically ill populations you can try the HADS. The reasons for choosing the appropriate instrument are that the population instruments are prone to ceiling effects in clinical populations (depressed people often produce very high scores on such instruments which limit discriminability) and the clinical instruments face floor effects (too few/too insensitive to milder, dysphoric mood) . Medically ill groups often have higher levels of vegetative symptoms such as fatigue or sleep problems which can artificially inflate apparent prevalence of intensities of depression in medical populations.