How do you find corner frequencies of differentiators or integrators?

You do this the same as for any transfer functions: figure out the limiting values for $\omega$ large or small (which usually involves an expression in either numerator or denominator of the transfer function), and then find the value of $\omega$ at which the values match. To take into account the intrinsic frequency-response properties of the op-amp, you would consult the manufacturer materials.