You can't. But the op had already hired scammers, and they were not meeting the productivity requirements. Or, you can try to guesstimate that by checking with

You can't. But the op had already hired scammers, and they were not meeting the productivity requirements. Or, you can try to guesstimate that by checking with previous employers. Which was the OP's point I guess, or alternatively the OP's point was "I give my employees a 40 hours/week contract, so they can't really have another job (and still perform adequately, or simply they can't depending on the law of the country)"