Watermark Recognation

Good evening to everyone,I am collecting the German stamps and have plenty stamps on the letters,some letters with thick and some with tin paper,lots of them with possibility with rare watermark,I have problem finding them and do not what metod to use to see them,please any help?thank you

On piece, always, is difficult. Best is to look at the back of the stamp, which is an issue with on-paper stamps. If the cancellations are legible, that might help. It is not fool proof, but a good indicator.
Otherwise, you will need specialised equipment.

Try scanning the stamps against a black background.

It sounds like you're asking about detecting watermarks when the stamp is still on the cover. There are several threads you can find by using the search function with different answers ranging from "nearly impossible" to suggesting different approaches that may help.

Here's one:

https://goscf.com/t/45526

Could you show a scan or two so people know what stamps you are talking about?

Even when the stamps are off piece, I still have difficulty sometimes seeing the watermarks with watermark fluid and a black tray though I am getting better at it with time and practice.

Thank you to all and for all,there are some stamps on cover/partially