| There are quite a few ways to do this...
1) make sure the salary to the employee is routed through you. The employer pays you and you inturn pay the employee (without taking a part of the sal)
2) The most common way to do it is, even before you give your employee details to the company, you come to an agreement with them stating that any reference of your they hire, they are liable to pay you for that. eg: You give them a reference of candidates a,b and c & send them over for interviews...later you find out that employee b has been hiried, since you have it in black and white (through mail or whatever) that you sent him there, technically he is your reference and the employee needs to pay you the cut. This method is pretty common.
Of course if the employee quits within a few weeks or months (Agreed upon time) you have to replace him and can't be charging for the replacement.
There are a lof of ways this is done...what I've mentioned are the simplest and the most common way of doing it. | 
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