Sampling Theory in Signal Processing
Incorrect initial statement.  The sampling rate must be greater-than (not just at-least or equal-to) twice the highest frequency present in a band-limited base-band signal, for aliasing (and thus potential information mixing/corruption/ loss) not to occur.

The maximum frequency present is usually by design, by putting a (estimated  or assumed to be sufficient) low-pass filter before the sampling mechanism.  (Or the mechanism of the signal or sampler itself includes such a low-pass frequency response.)