leeks or onions

What about squeezing a shoot you should find the onion has a characteristic smell.
Can’t think of any other certain method when they are small, although I have noticed that my very few onion seedlings seem to flop over more than the leeks.
Good luck HH

As the seedlings develop you'll be able to see from the multiplying leaves whether they're one or t'other. Leek seedlings develop flat leaves, whereas the onion leaves remain tubular.