I do not think an exam, length of membership, or number of posts should be the basic criterion for evaluator status . The moderators Know who are the proper members to undertake this. If a member indicates willingness to participate the moderators can decide on suitability.
This would establish a pool of qualified reviewers .

If a snippet is tried and fails for a member and it appears that it should have worked the moderators could go to the pool select from the best suited members and request a review of the code.

All pool members coud review any code but I suspect that those best suited to the task are also tose with the least free time. Being selected or not for the pool would be private between the moderators and the aplicants. If an applicant is declined it would be unknown to the general forum and the reason would be given by the moderators to the aplicant.
I feel for Craig and Steve asthis site grows there will be more work thrust on them. Thanks guys Terry