No matter how diligent you are in labeling your seed trays, a few unidentified plants are always left over. You may have to wait until harvest time to determine the difference between an orange bell pepper and a yellow bell pepper, but you can use a few tricks to tell the tomatoes from the squash and the lettuce from the beans. In the meantime, keep the soil evenly moist and provide enough light and nutrients so that your vegetables can continue growing.
Make a list of the seeds which you planted. Check off names as you identify the vegetables and use the process of elimination on any unknowns.
Check your seed packets for germination times. Some packets will also contain a description, or even a picture, of the seedlings.
Look for fuzzy hairs along the stem. These should be tomatoes or peppers. Tomatoes will be fuzzier than peppers and have fern-like leaves.
Consider the shape of the leaves. Squash seedlings from zucchini to yellow crookneck will have thick, rounded leaves. Carrots will have fine, fern-like foliage.
Compare the appearance of possible lettuce and spinach seedlings to the adult plants. These seedlings will develop the ruffled leaves and green to purple coloration of their specific varieties quite soon after germination.
Compare any seeds which come up with the seedling to the remaining seeds in the packet. Bean seedlings, for example, often have an easily recognizable split bean seed attached.
Take pictures or make drawings of your seedlings. At some point, it will become obvious which vegetables these are. Store and label your images as a reference for future years.