Photograph the whole surface
A clear picture from a safe position can show the location and surrounding frame. Do not press on the glass or test cooking zones to judge the damage.
Identify the model first
Glass availability and construction differ. Model details help establish whether repair is feasible; a similar shape or printed ring layout is not sufficient to select a replacement surface.
A useful distinction
Damage near a control area may be less conspicuous than a line across a cooking zone. Photograph the whole surface and identify the visible crack without using the hob to determine which functions still work.