How to justify an instrumental variable?
To motivate a good IV in a paper, you need a good story. For example: schools build in Indonesia as an instrument for education outcomes, quarter of birth as an instrument for income as a result of compulsory education, in their respective papers.

The IV has to be correlated to the characteristic you're trying to fix and uncorrelated to any of the other observable characteristics. 

That way, the IV is only affecting the Y variable through the characteristic you're instrumenting. As far as statistically strong, you can look at examples from a few papers and see what kind of statistical scrutiny they used.