Apparently most distributors of halal meat to the restaurants / takeaways supply what we call mutton, and they call sheep, which has a stronger flavour than lamb. (Certainly, that's the case in my local wholesalers, which supplies some of the local restaurants with meat. I occasionally go halves with my brother on a whole animal, and when I initially asked if it's lamb or mutton I get a quizzical look and the same answer: "Is sheep".)
But while you should be able to taste the difference between beef and mutton/lamb, even in a hot curry, if the beef has been cooked in a rich lamb or mutton stock then it's definitely going to be more difficult - especially if you're not looking for it.