Answer:


Measure distributions in a series of known, monomeric, cytosolic or extracellular proteins. Score. Get distribution, callibrate to 0.0 +/- σ and set limits at +/- 4σ. Check by running a few unfolded proteins and a few membrane proteins. Those should fall outside the limits.

Distributions are measured, for example by measuring the accessible surface areas of all hydrophobic and all hydrophylic atoms that are at the surface, and dividing the two sums (other algorithms can be invented easily).