My dear @Jolita, a numerous answers were given under following thread and I think it could help you in research! Almost 200 answers were delivered!

https://www.researchgate.net/post/Can_anyone_suggest_a_validated_instrument_for_measuring_stress10

The Depression, Anxiety, and Stress Scale (DASS). It comes in different forms, like a 41-item and 21-item questionnaire. So you can either go more in-depth or have the participants save time when fill it out. 

It can be used in both clinical and nonclinical populations. And is used in multiple countries. 

See:

The structure of negative emotional states: Comparison of the Depression Anxiety Stress Scales (DASS) with the Beck Depression and Anxiety Inventories

The short‐form version of the Depression Anxiety Stress Scales (DASS‐21): Construct validity and normative data in a large non‐clinical sample

Psychometric properties of the Depression Anxiety Stress Scales (DASS) in clinical samples

And what about physiological measures, e.g. hormone measurements? Why are they rarely applied? Too expensive?

My dear @Jolita, a numerous answers were given under following thread and I think it could help you in research! Almost 200 answers were delivered!

https://www.researchgate.net/post/Can_anyone_suggest_a_validated_instrument_for_measuring_stress10

Regarding available and validated instruments for measuring the stress in social sciences, please do find attached tools! Perceived Stress Scale (PSS) - first link! The second link brings many available instruments for different stress measurement, as few next links bring concrete examples of those tools!

http://www.midss.org/content/perceived-stress-scale-pss

http://www.midss.org/taxonomy/term/27

http://www.midss.org/content/organizational-police-stress-questionnaire-psq-org

http://www.midss.org/content/measure-current-status-mocs

http://www.midss.org/content/interpersonal-support-evaluation-list-isel

Three levels of factors affect stress:

1. Individual based factors

2. Organizational factors

3. Environmental factors

Better to devise your instrument covering above factors by consulting various instruments available and literature, as stress variables may differ due to culture variations, place specificity, organization specificity, different environment, etc.

I would recommend Cohen's Perceived Stress Scale if you are wanting to assess the perceived stress experience by the research participants. This is one of the most widely used instruments in assessing perceived stress.

This link also has access to a free copy of the scale: http://www.mindgarden.com/products/pss.htm

Here are some readings on Cohen's scale that might be useful too:

http://www.jstor.org/discover/10.2307/2136404?sid=21105288842061&uid=2460337935&uid=3738640&uid=63&uid=4&uid=2&uid=2460338175&uid=83

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1756880/

You you can also find more on the psychometric properties of this scale here:

http://onlinelibrary.wiley.com/doi/10.1002/j.2161-1882.2006.tb00100.x/abstract

The following website has information about some stress-related instruments and links to other resources about instruments.

http://shell.cas.usf.edu/~pspector/scalepage.html

The national survey of health in Chile uses the perception of stress from  INTERHEART. This is the first major international study to investigate the primary causes of heart attacks, worldwide

A similar (but not identical) question was asked in Research Gate in September 2012 (Please, see the link below). My comment is: There is no universal agreement about an exact definition of stress as applied to variety of persons with different ages or backgrounds or life experiences. Human beings can be subjected to some measurements which give reliable results (such as body temperature, blood tests...) but there are measurements that cannot work with validity (such as depth of emotions or feelings). The human being has complexity & can change, modify, pretend, cover up ...etc. under direct tests. Also, some human beings may have developed a high degree of tolerance against recurring stresses so their response will be the famous saying (everything is fine). I think that Depression Anxiety Stress Scale-21 (DASS-21) (may) work but at a close estimation, no more.

https://www.researchgate.net/post/Can_anyone_suggest_a_validated_instrument_for_measuring_stress10

An indirect measure of stress can result from the possibility / impossibility of controlling the symptoms of functional disorders like irritable bowel, gastro esophageal reflux, functional dyspepsia. The epidemiology tells us that these disorders are growing exponentially and that greatly affect the quality of life.

I am very grateful to all of you for sharing the information. As I am only starting to go into this new area for me, I really need help and advice. Your recommendations and links are very useful.

Multitrait scaling analyses supported the dimensionality of four functional support scales (emotional/informational, tangible, affectionate, and positive social interaction) and the construction of an overall functional social support index. These support measures are distinct from structural measures of social support and from related health measures

MOS Social Support Survey enclosed.

http://cmcd.sph.umich.edu/assets/files/Repository/Women%20Take%20Pride/The%20MOS%20Social%20Support%20Survey.pdf