What you're looking for is somebody like my son (yeah, he's already got a job). He has built/resurrected a few cars, has a math degree, played college baseball, and is married (yeah, you can't ask that, but...). With those skills at a time he needed a job, he went to work for a pump company (sewer type). He got the job through a guy he played beer league baseball with not because of any of his qualifications that you would find on his resume. His boss knew he had the aptitude, BUT the personal relationship is what counted. It wasn't the ideal job (too many family members of his boss also working there). His current job was based on the success he had at the previous job. He is now working on irrigation pumps and controls. His boss asked him where he learned to work on the controls. He told him it was on the job.

So, look for aptitude and attitude. Ask about outside interests and accomplishments. Also, understand that for every person like my son (who is now 37) you will encounter 50 who won't perform at a high level, maybe more. Being c0ommercial HVAC, try to read physical condition (read college athlete). A high level math and science former student also has the ability to learn.

Hiring has always been a crap shoot, but learning to read people will help.