There's a 'porthole'/dirt ingress system to look through. If you can see seat post, you're at safe insertion, so it's easy to be sure. Henry's looked right when I last saw it.
It's a little hole so you can see through the frame and check the seatpost has gone in enough. Looks like there is a clear sticker over it, but as soon as that falls off, its an ingress system.
There's a 'porthole'/dirt ingress system to look through. If you can see seat post, you're at safe insertion, so it's easy to be sure. Henry's looked right when I last saw it.