My first thought: You can modify a self-assessment to suit your purpose. Just change the instructions and edit the wording of the items so that it refers to the respondent's judgment about another person. 

There are personality tests that have both self-report and other-report versions, with essentially the same questions on both forms. This is not a weird thing to do.

Of course, you will need to establish the reliability of your modified measure. But you should do that even if you were using the original, really.

I agree with Stephen Joy, in terms of approach... additionally, have you had a chance to look at the discussion initiated by Merylle Mathew https://www.researchgate.net/post/What_are_the_different_scales_to_measure_emotional_intelligence.