It can be hard to tell for sure if you're on something that suppresses symptoms (i.e., birth control), but for me I went through a really horrible 6-7 months where I was very sick with endo symptoms and my doctor said that was probably my endo progressing.

When u start having new symptoms that happen more oftan than before. Other than that if u dont experience symptoms the only way to know if it progressed is through surgery. Its said that pain doesnt really correlate with whats really going on. So u can have no symptoms and have alot going on inside u didnt know