pippi21, the way I learned was by watching my own plants. Leaving the flowers on, letting the seed heads develop and then when they look ripe, checking, and maybe rechecking. Also these forums are an excellant resource. If you have a particular plant you want to save seed from, ask about it, and I am sure someone will chime in with what to watch for. Also check out this fantastic site below. You can look up pictures of seeds and seedlings for many plants.
Tammy

Here is a link that might be useful: The Seed Site

That will be a big help in Spring and Summer. Thanks!

Hi Pippi,

If you look at the Seed Saving forum here at GW you will find the bulk of my seed saving FAQs.