Summarize:

Look for something that looks like a tiny onion at the base of a green stem (which fades from lime to emerald green upward from the bulb). The stems are usually thicker and sturdier than green onions to support the bulbs. Spring onions are a more mature version of green onions, hence they have a larger bulb. Most markets and grocers sell spring onions with the bulbs attached and the tops of the stems trimmed down to about 4 inches (10cm) to 6 inches (15cm) long. They'll often be sold in a batch of 5 or 6 bulbs with the tops attached. True to their name, spring onions are harvested in the spring, so depending on where you live, you're more likely to see them at farmer's markets around those times. If you notice there are small bulbs that look similar to garlic at the base, those are shallots. They grow in tight clusters and typically look a little like small brown onions, only more elongated. You'll be able to spot them because they aren't sold with the greens on top. Shallots come in purple, brown, and gray varieties (the gray ones are especially rare and considered to be a French delicacy). Shallots are harvested in summer and fall, so that's usually the best time to eat them. You can also give them a sniffthey have a zingy, sulfuric smell.
Identify spring onions by their long green stems and white bulbs. Notice the bunched, garlic-like clusters that are a hallmark of shallots.