This is a complex area. To ID your stamp there will be several screens whereby you select a stamp characteristic. Click the characteristic that most closely resembles yours
Small crown watermark
Large crown type I watermark
Large crown watermark
type II
No spikes in the centre
HOW TO DETECT WATERMARKS
Sometimes the watermark is visible to the naked eye by viewing the reverse of the stamp. Laying it face down on a black surface may assist, or sometimes holding the stamp to the light. In more difficult cases one needs watermark fluid and watermark tray. These inexpensive items are available on the internet.