More typically, you limit the amount of rent increases for new tenants, and have the original reference for rent be set before the law was brought up, so nobody had time to preemptively raise rent to cover rent control.
In the Netherlands there is a points system. Every feature of a house like the number of bathrooms, windows, heating, etc contributes to the score. It's mostly used to define the social housing threshold: houses below a certain number of points are removed from the free market and have the rent fixed as a function of the score.
For houses on the free market, you look at average price per sqm for the area. High rent is off the curve. You can also correlate to the baseline set by the points system, and rent as a proportion of average income to define 'high'.
Some rent control laws only apply to leases of individual tenants and don't apply between tenants. In those, as long as someone leaves 'normally', you can charge the next tenant whatever you like; although if everyone sets excessively high rents to counter rent control, exvessively high rents are the market rate.
Other rent control laws do limit rents since the start of the law or sometimes a point in time before the law.