how do one knows where the moisture from tat cause water vapour on testing equipment like multimeter, oscilloscope? Please advise.

 

????????

What are you asking?

 

if you put your multi meter or scope above a pan with boiling water it comes from that

 

I think he or she is asking for an instrument like a hygrometer to measure relative or absolute humidity.

 

I think you're right, if so, a humidity sensor of some sort would fit the bill.

Here's a temperature/humidity sensor.
http://www.parallax.com/Store/Sensor...%2cProductName