There is no technical measure.

Instead, there is a heuristic that makes them curious about the purpose to which you are putting all your data usage. They then have the right to ask you to show them what kind of stuff you're putting in the cloud drive, at which point they can apply the actual measure—a human, ontological, qualitative measure of the "color" of your bits.

This is how the legal system works; this is how contracts work. Their provisions don't have to be interpretable by a dumb algorithm; they can be AGI-complete to solve for. This is why the court system has both police (instructed in an simple-but-false-positive-generating heuristic), and judges and/or juries (who are expected to then apply the human-deliberation-requiring "true algorithm" to screen out the false positives.)

Picture a savings account that holds "unlimited money." Does this mean that it holds money derived from tax fraud? No. Does this mean that it holds money used to create a correspondent deposit account for a wire-transfer service? No. These facts are intuitively obvious. Why doesn't it do these things? Because an account that does these things ceases to be a savings account per se. In the former case, it becomes an illegal conspiracy if they knowingly continue to serve you. In the latter case, it becomes a business account requiring strict money-service auditing. In both cases, if the bank detects suspicious activity (i.e. pushing millions of dollars around every day in said account), they have every right to ask you to what purpose you're putting their service—and to take the service away if you refuse to answer.

"Show" them? As previously mentioned, I'm encrypting my content. The nature of the data I'm story is both none of their business (I'm paying for unlimited data storage, not unlimited data storage of certain file types after all, per their advertising) and anyways it exposes them to further legal liability by way of asking that question.