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My wife had to deal with many such cases when they were hiring contractors in their group. A few simple checks that could be useful:

1. Request to have some date of birth, graduation date etc on the CV. It's by no means fool proof, but it at least prevents a 23 year old passing a person with 10 years of work experience.

2. Replace phone interviews with Skype interviews. She says many times they can actually see the candidates googling for answers and reading the top most result. Of course, they never hire such a candidate.

3. Maintain a company-wide data-base of who is hired/interviewed from which consultancy and if the there were any red flags. This will help other groups in a big company. Since there is a written record, people also tend not to interview from the same bad company again and again and wasting everyone's time.