This is generally not an indicator available for young people (and I think it's reasonable for MIRI recruiting efforts to target young people), and when it is, it isn't obviously a good idea to use. There are research opportunities available at the high school and undergraduate level, but they are not universally available, and I have been told by people who sit on graduate admissions committees that most of the research that gets produced by these opportunities is bad. Among the research that is not bad, I think it's likely to be unclear to what extent quality of the work is due to the student's efforts or the program / advisor's. (I have heard rumors that in one particular such program, the advisor plots out the course of the research in advance and leads students through it as something like a series of guided exercises.) Edit: I worded that somewhat poorly; I didn't mean to suggest that the work has been done by someone other than the student, but a nontrivial portion of the success of a research project at this level is due to the careful selection of a research problem and careful guidance on the part of the program / advisor, which is not what we want to select for.

By contrast, tests like the AMC (which leads to the IMO in the US), the Putnam, and the SAT are widely available and standardized.