Do a Google search for a weed ID site.
My motto is it can stay if I like the way it looks. Once it looks weedy or interferes with something I like better, out it goes.

What I use, and practically can't live without now that I have it, is a book called 'Weeds of the Northeast', put out by Cornell. I've seen it at Barnes & Noble recently. It's photographs of a lot of different weed seedlings and mature plants. It's pricey for a paperback, so you may want to look at it first. I ran into it at a local garden center. I was doing a rose education table next to a turfgrass guy. He had a bunch of weed books, and I liked this one the best.

Very interesting guy to talk to. I learned a lot about lawns that day.

i dunno... i'm taking a wait-and-see approach. if you're not certain, i would say that you should leave it until you know whether not you want it to stay there. this is my second spring with our garden, and i'm getting better at recognizing what was planted, versus what is unwanted. but, when in doubt, i leave it until i'm certain.

I have to vote with the others. I still have an incredible knack for yanking away the wrong plants. The perennials do tend to continue their growth in the weed pile, from where they're rescued and put back in their proper spots. Extra work in a tight schedule, that!

A search on http://dogbert.abebooks.com resulted in several copies of "Weeds of the Northeast" by Cornell University Press. Least expensive was just under $25. I'm ordering my copy tonight. Thanks for sharing that with us, Mad Gallica!

H

Here's a website with some pictures of seedlings.

Here is a link that might be useful: website