When it comes to feral/naturalized examples of the cultivated varieties, onions will always have round hollow leaves, while garlic and leeks have flat leaves with a crease down the center (sort of like a "V" in cross section). Garlic and perennial leeks (including elephant garlic) can be confusing. The best way is to look at the mature bulbs in the summer. Leeks will usually have little offsets ("corms") around the base of the bulb that are on thin stalks and separate easily. Garlic will not.

When it comes to variously named wild species of garlic/leeks/onions, that's a whole other question.

Some pictures might help.

The weedy wild garlic is thinner and darker green than cultivated garlc, with a very long shank that leads 6-8 inches down to a half-inch diameter bulb, often surrounded by hard little pointed corms. It takes me about five years to get all of it out of a new bed.

I have some hardneck garlic gone wild, too, in an uncultivated area where I dump shrivelled bulbs. It comes up in clumps with flat, very dark green leaves. Right now it's about 8 inches tall. This time of year I dig them and use them like scallions.