Goertzel algorithm: Relationship of magnitude
You can potentially base the threshold on the overall energy in the frame. If the tone has, say, 10% or more of the total energy, there is a good bet it's there and not just noise.

In general FFT based algorithms may not be the best choice for this. Unless your tone and your local sample clock are phase-locked, you can't line up the tone on one of the FFT bins (or your Goertzel frequency) so the energy of the tone will "smear" out over multiple frequency bins. The FFT is actually a pretty band band pass filter. 

The alternative would be a time-domain band pass filter with a well defined bandwidth and steepness. This would also get rid of the framing constraint and you can dial in the time constant based on the properties of the signal rather then internal properties of the algorithm.