I usually watch the birds. When then are going after the seed vigorously then I try and beat them to the remaining seed.

Googling for ~ harvest sunflower seed ~ I see at least one other person takes the 'watch the birds' approach.

Hmmm, the Finches in my yard start eating my Sunflowers seeds while the petals are still yellow! I don't bother collecting the seeds. I have sunflowers coming up all over the place every Spring. I just transplant the babies if they need to be moved. I let Nature do most of the work!

If you are afraid of collecting too soon, put a pantihose leg over it. Once you see seeds coming out of the flower head, you can harvest.
Remy

Thanks everyone. I'll be anxious to see if I get some plants next year as you have tasymo. Off to find an old pair of pantyhose.