Read the opening and rim measurements
Distinguish the usable opening from the overall head.
Start with the opening
Measure descriptions can refer to the usable opening or the outside width of the head. Ask which figure is listed and look for a front view with a scale.
Check whether the head detaches
A removable head can be cleaned separately as directed. It does not guarantee that a replacement is available or that heads from another generation will fit.
Compare the mechanism
Air pulses and a physically moving tip are different designs. See the moving-tip comparison if the listing names both.