There is a reason financial accounting is a degree in most Universities. There is a lot to learn. I suppose your options are to hire someone who has such a degree, hire someone who has the relevant experience (even if it's just on a temporary consulting basis), or study that field yourself (even if it's just a couple of courses you take online on business management, accounting, and auditing).
With that being said, here are a couple of suggestions:
Know your biggest clients. Track their purchases over the years and over the months on a spreadsheet. If restaurant or cafe owners changed their spending habits with your store, or that they don't correlate with the spending patterns of other restaurants/cafes. Try to reason why.
Know your employees. If some of your employees seem to live above their means. Try to figure out why.
Invest in high-resolution motion-activated cameras that are aimed at the scales and at the cashier, and that store the footage for a long time. That being said, the bigger those files and the longer you need them stored, the more expensive it's going to be, which brings me to my next point.
Implement more frequent inventories and spot audits.
Train all your employees in being able to spot the cut of the meat and therefore roughly the quality/price of the meat by just glimpsing at it. You can make a game out of it.
Institute an employee reward system, for catching large mistakes and/or for catching internal thieves. At UPS in the US for instance, their reward is $5,000 for catching employee theft. The amount would actually vary, but an employee would only get the full amount if his information helped in the arrest and in the successful prosecution of a fellow employee (and of course, security personnel and managers are exempt from being able to receive such a reward since it's already their job to catch the bad guys).
Implement a reward system for finding security flaws and generating potential solutions. After all, why pay a consultant to tell you where the weaknesses are in the system when most likely, your own employees already know where the weaknesses are and possibly know how to fix them themselves.
And obviously, you are not located in the US since you stated the quantities in kilograms. But wherever you are, just know that theft-prevention is highly dependent on the local laws that you have at your disposal, so not knowing your local laws, that really limits the kind of suggestions we can give you.
And ultimately, you need an expert in theft prevention and proper financial controls, whether it's you or your father becoming one, or someone else you hire who is already one.