Asad, if you are literally talking about authenticating the candidate to be sure the candidate on the resume is the same person (candidate) on the phone screen and is again the same individual on the final skype call and is the same person who shows up to perform the work i would say this:
1. the candidate and/or his/her 'handler' (often the company sponsoring the candidate's visa if we are talking about foreign workers being screened and hired for temp IT projects here in the US - please clarify) are crooks and they should be reported to the authorities.
2. consider starting a service (think "eTrust" for recruiters) or an association of recruiters whereby you and the members can help identify and weed out the unscrupulous agencies/handlers. I will charge only a small equity fee for the idea :-).
3. money is the motivator here so the most effective way to protect the customer is via an effective contract that provides for financial penalties for fraud/misrepresentation.
4. The candidate and his/her handler need to be notified from the start that fraud or dishonesty will not be tolerated. Having the handler provide the candidate's visa photo and visa ID# at the start will put them on notice.
5. Put a financial burden of proof on the candidate's handler. That could be, for example, the handler is responsible for travel and other expenses and the first 14-30 days wages for the candidate which will be reimbursed to the handler upon confirmation that the person who shows up for work is the correct person. If not then the person is sent back with no financial obligation other than the handler being responsible to pay the candidate's wages, taxes, etc.
5. I would consider requiring that the handler provide a copy of candidates visa photo and visa ID# with the resume and all subsequent interview steps should be via skype so the interviewer can make a visual confirmation. This doesn't address the use case where the resume does not belong to the person who shows up on camera, but this is addressed in the contractual terms and the tech screener should be able to ferret out any discrepancies.
6. with the visa ID# any confirmed or suspected fraud can be reported to ICE (immigration and customs enforcement). The handlers are often on visa's themselves and if not they still understand that they can be tracked down as the visa sponsor.
7. don't forget us little guys when you strike it rich!