How to tell if they've breached the cap? Wait outside their offices and watch for carloads of expensive lawyers arriving... :)
The sticky issue for me is this: why should two individuals not start a business venture together? The fact that one owns the club and the other plays there may look a bit dodgy, but at the same time that shouldn't preclude them from going into business. It may well come down to how profits are shared out, whether funding was put in place by one party and the relevant paperwork legitimises this, whether the correct tax is paid etc.
It doesn't look good. But that doesn't mean it isn't legit. No doubt the club have been very careful in how they present this to the governing body.
There is nothing stopping them from going into business together whatsoever. The point is that any and all additional benefits (even gifts) all have to come under the salary cap. So the real question is whether or not they have declared these business ventures to PRL and included them in expenditure, as they should have.
I'd hazard a guess that they haven't. No doubt that won't stop them from getting away with it.