How to compute input impedance?
Calculating the input impedance by hand is almost certainly what you're supposed to do as the other answers have suggested.  I just wanted to show you how to go about getting some numbers out of a circuit simulator so you could check your work (or apply the same concept to a more complicated circuit).  Here's your Sallen-Key filter in CircuitLab:



And here's the frequency domain simulation showing the input impedance looking into the input:



You can open the circuit and change the parameters, configuration, op-amp model, etc.  Just hit F5 and you'll see the V(out)/V(in) Bode plot, as well as the input impedance plot that I've included a screenshot of above.  Using custom expressions in the simulator, like MAG(V(in)/I(R1.nB)), allows you to calculate quantities like small signal impedances quite quickly!

Using a test current source, rather than a test voltage source, makes sense for how I'd probably go about solving this on paper.  However, for simulation purposes, using a voltage source as the test input allows us to more easily understand the V(out)/V(in) Bode plot at the same time.