If a fund has 30% active-share, only 30% of the portfolio can differ from the index. A 1% annual fee charged on the whole fund is effectively a 3.3% fee on the active portion, a hurdle very few managers clear.
The pattern usually comes from career risk rather than dishonesty: a manager who deviates and underperforms gets fired faster than one who tracks the index and underperforms slightly. Risk controls that cap tracking-error can enforce the outcome even when nobody intends it.
Detection is straightforward with holdings data: compute active share, compare fee to the index alternative, and check whether stated deviations persist across reports. Several European regulators have published reviews naming funds on exactly this basis.
Related: active-share, tracking-error, management-fee, index-fund, fee-drag, benchmark