How do i know if i've correctly placed my diaphragm?

Shouldn't feel it: A diaphragm should fit so that is completely covers the cervix and tucks (witch hazel) behind the pubic bone. You shouldn't feel your diaphragm if it is correctly placed. This includes after squatting, sitting, or walking around. If it slips doing any of these things it is not correctly placed.

I suggest: You go back to the doctor/office/provider who fitted and prescribed and ask him/her to check your placement. It's not hard but you must get it right or it doesn't work. One short appointment will give you confidence.