I've been keeping an eye on my hairline since October but the only way to be sure would be to take some photos (or better still measurements), from say the eyebrows to the start of the hair. I've got a widow's peak so I'd measure from that and from the areas beginning 5cm to the side of it which at this point in time still resemble the female pattern. Those are the parts I'd lose noticeably if I was losing any, as well as the tip of the widow's peak.
So far there seems no change to the hairline on T. I don't think there is any dramatic line because the changes occur slow enough and at the edge of the hair the skin quickly becomes like the rest of your face (same if you trim your hair quite short all the time, it doesn't look like a scalp that's never been exposed).
It's slightly tricky for me to keep track of my hairs because some of them are lighter and some of them darker... the ones at the edge of the hairline are always much lighter than further in, although T seems to be altering this in the same way it's made my arm and leg hairs darker than they used to be.
What they mean by hair starting to match the natal male pattern is kind of like in this pic

The area where the woman has hair circled by red will start to fall out matching closer the man on the right. As guys get older that pattern then tends to recede backwards widening these areas without hair, although how far back they go and whether you get a bald spot on the back is probably down to your genetics more than anything.