You could use sterile dirt and then you'd know that anything sprouting is what you planted.

Also, there are a couple of website that have extensive seedings pictures. I don't remember the details, one's in the UK, maybe 'the seed site' and tom clothier might have this as well. You could google for those and other sites, or even google for images of the seedlings. Does that help?

I had a similar quandry recently, I was sprouting Golden Berry aka Cape Gooseberry (I guess, I don't know, it's new to me) which I've read are related to ground cherry, maybe even the same thing, can't remember. Even though I used what I thought was a sterile medium, I had two different things sprout. I let them both grow until I could for sure identify the one as the weed, and assumed the other was the Cape Gooseberry. It doesn't look anything like a tomato, and if I remember correctly from my youth, ground cherry leaves don't look like tomato leaves either, but I'm not sure. Not sure if that's helpful at all.

Seedsilly

Here is a link that might be useful: Physalis peruviana L.

Thanks for the help.

I've got some things growing in the approximate places I put ground cherry seeds (rows in small starter tray), and they look very like the nightshades that grow wild and most likely contaminate the barnyard dirt I used. Although I might be mistaken on the nightshade thing; I usually don't pay much attention to them until they get big and I decide to pull them up and get rid of them.

One or two of my prospects are quite robust and need transplating PDQ; the rest are quite small. Time will tell, I guess...