Sure you can know
Mostly from guestimation. When I was deworming I believe I have killed at least one (that was when I was taking a dewormer as scheduled, periodically taking little amounts of garlic with vegetables thenafter papaya seeds) because the proglottids are fewer now. I remember biting into an uncooked pork sandwich years ago.
The segments are actually very small but I see them when they are tinged brown. They are maybe 1mm (sometimes maybe 3mm) long and ~.2mm wide. Very small.
I am getting sick of pineapple now. The one I am on now was kinda green when I cut it and it burned my tongue a little, that possibly means greener pineapples have more bromelain. Maybe the bromelain breaks down the starches into sugars (I read papain does that to papaya, as unripe papayas are said to have more papain). I eat the core.
Anyhow, I will start my 4th pineapple tomorrow. Not looking forward to it, I am very hungry for real food. It was delicious when I started.