You can use principals of calculus to differentiate a function and then implement the derivative in blocks in Simulink. For example, the derivative of f(x) = x^2 is 2*x. So you would just multiply your input value x by 2 to get the derivative.
Or if you are talking about numeric differentiation, that might be a lookup table. If you have y = f(x) as a table of numbers, then you can approximate that derivative of f(x) using two lookup tables in Simulink and the approximation dy/dx ~ (f(x+dx) - f(x))/dx. On second thought you probably would just make a new lookup table for dy/dx as a function of X and then have Simulink use that.
