The scam is that somewhere in the literature you are to cash the check, take out your pay and then forward the balance elsewhere. The check is bogus and yet looks good. It goes through your bank and by the time it comes back as being fraudulent you have already sent along their money (your money). You now have trouble with the bank for cashing a fraudulent check, you have worked a job that was bogus and you have shipped out your own cash never to be seen again.
It is not the way mystery shopping works. First you register with legitimate companies, then you request or self-assign jobs of interest. You then perform the job, submit the report and wait anywhere from 2 weeks to 3 months to get paid for your work. Usual pay for a bank visit is $5-$20 and I'll bet you the scammers claim they are going to pay you $100 or more.
So be serious. Have you ever had a job where they gave you money before you did the work?