How can I detect them?
Search this forum for watermark detection.
Watermark fluid is available from stamp shops.
Others use good quality Lighter Fluid (Obviously caution is required using flammable liquids) Just dip the stamp in and place on a clean black background (dish)
You will need a perforation guage available on eBay
Stamp collecting 101
When requesting a stamp ID, students should endeavor to offer stamp perforation, any watermarks detected, and the users Catalogue Source (Stanly Gibbons, Scott Michel etc)
Enjoy your new hobby