Two elements are needed: control over the account, whether formal discretion or de facto influence over a passive client, and excessive activity given the stated objectives. The quantitative evidence is an annualised turnover ratio and a cost-to-equity ratio showing what percentage return is needed simply to break even after commissions.
Rules of thumb used in finra-arbitration treat turnover above six and cost-to-equity above roughly 20% as strongly indicative, though context matters for an account genuinely intended for active trading.
The modern variant is not commission churning but excessive recommended trading in high-cost products and options, which the quantitative limb of suitability and the care obligation in regulation-best-interest both reach.
Related: suitability, regulation-best-interest, finra-arbitration, fiduciary-duty