From the point of view of positive psychology, the subjective wellbeing scale developed by the World Health Organization is an ideal tool to understand the mental health of an individual.
I have recently published an article in a journal published by Routledge: Taylor and Francis, where I have used this tool https://doi.org/10.1080/15524256.2022.2112808
There are so many tools used for measuring the level of mental health from the perspective of positive psychology like:
Subjective Well-Being (SWB) Scales: These scales assess an individual's overall life satisfaction, positive emotions, and sense of meaning and purpose in life.
The Positive and Negative Affect Schedule (PANAS): PANAS measures positive and negative emotions experienced over a specific time frame.
The Authentic Happiness Inventory (AHI): This inventory evaluates three dimensions of happiness: pleasure, engagement, and meaning.
The Flourishing Scale: This scale measures overall psychological well-being and how individuals are functioning in life across multiple domains.
The Strengths and Virtues (VIA) Survey: The VIA Survey assesses an individual's character strengths, which are integral to positive psychology. The Gratitude Questionnaire (GQ-6): This questionnaire assesses the frequency and intensity of feelings of gratitude, a positive emotion associated with well-being.
Referring to different scenarios related to well-being of an individual, I have come to accept that a multidimensional approach to mental health offers a better understanding of an individual’s mental health status. For instance, in an academic institution and its related challenges, a student could be better understood and helped by considering a variety of the situational factors that have a bearing to his/her mental health. I find this link very useful to address the question posed. https://www.researchgate.net/publication/268924012_A_psychological_instrument_to_measure_university_students%27_psychosocial_problems
Many tools can be used depending on the aspect of positive psychology you are considering. Example, Marital satisfaction for marriage, career satisfaction or adjustment for career, academic achievement test for academic performance, self awareness/ acceptance for emotional intelligence,etc.