You seem to have a few misconceptions here.
First, an input capacitance of just 10pF is very unlikely to be created by a discrete 10 pF capacitor. Rather it will arise from stray capacitance of the input wiring and of the input device itself. In reality, you may well find input capacitances much greater than this as capacitors are often added (normally 100 pF up to 1000 pF) to prevent RF interference.
Second, the 1MOhm and 10pF do NOT form an RC filter, obviously or otherwise. What DOES create a low pass filter is the output impedance of whatever is driving the input and the cut off frequency will be determined by the output impedance of the driver, the input impedance of the input and the impedance of the cable connecting them. Hoping to explain this a bit better, consider the case where the driver output impedance is actually 0 Ohms. It does not matter what load you put on this as there will be no loss of signal, hence no filter action. Of course in this case, input currents could become very large but who cares if the source can drive it.
So on to the real answer to your question. To determine the the cut off frequency of any filter formed from output impedance and input impedance you must do the proper calculations. There is no way around this if, for instance, your input is from a passive guitar. The output from this itself will be a reactive impedance and the calculations depend on good measurement of the guitar. Sadly, as you move the guitar volume and tone controls, the output impedance will also vary. Altogether a quite complex scenario.
If we consider the output from a pre-amp though, certain approximations can be made. Output impedance can usually be assumed to be purely resistive in the audio band and this can be quite easily measured and is unlikely to vary. Hence we would expect a figure of a few 10's or 100's of Ohms. Then add the input capacitance of the input stage to the capacitance of the cable (Yes, they sum at all times). The frequency where capacitive impedance equals the source impedance is your 3dB cut off frequency. This approximation assumes that the input resistance (1MOhm in your example) is very much greater than the source impedance.
Hope that helps.