The picture above shows spinach as it has just sprouted. The little seedling to the right of the left hand spinach looks a lot like a beet, but it's the seed leaves of a related plant called variously lamb's quarters or pig weed (chenopodium album) which is my first harvest after the volunteer mustard. The little plant with the round lobes below the spinach is ragweed (and there's another tinier seedling with yellow-green round leaves, which is also a weed) It looks a little bit like a lettuce seedling right now. One of the ways to tell the difference is to look around and see if the seedling appears in the path, in other plots and the path. Then you can assume it's probably a weed.
These are the seed leaves of radishes, but this is also what any of the coles (cabbage family. think coleslaw) would look like. So if you've planted broccoli from seed, or kale, or bok choi, cauliflower, brussels sprouts or mustards, this is also what the seed leaves would look like. The seed leaves are also called cotyledons, and they generally do not look like the "true leaves" that come next and grow and nourish the plant. These two seed leaves were part of the seed itself, ready to unfurl when it sprouted. This is easiest to see in a bean, which easily splits apart into the two cotyledons.