I have the same problem. I use a couple of things to help. I look up what I want on the site below to get an idea of what the seedlings I want should look like. Then if I don't know the plant I post a picture on the 'Name That Plant' forum. Of course after a while I have gotten to know some weeds right away like partridge pea and lespedeza.

Hope this helps
JP

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

Growing up on a farm, I learned most weeds while I was a toddler imitating mum pulling weeds in the garden. Nowadays, I mostly use Weeds of Northeast book for the newcomers -and between birds and the wind, newcomers pop up all the time.

Here is a link that might be useful: book on Weed identification

I guess the answer it to learn what they look like....and what the plants you planted look like.
many years ago when I first started gardening, I would get "in the zone" when weeding and was so programmed to pull weeds and leave plants, that my gardens were soon over run with all those things that self seed so readily.
I believe I have finally learned that a weed is just a plant out of place!
Linda C