You'll be able to tell when they're much older. For one, the lemon leaf will smell like lemon when crushed. You don't want to crush any leaves at this point! Also, the lemon will probably outgrow the calamondin -- and maybe have larger spines?

You'll also probably be able to tell by the petioles once they have a set of true leaves.

I gave away my lemon seedlings but here’s a pic of a calamondin seedling. A couple months old. The calamondin will probably have shorter Internodes than the lemon.

I would think the lemon seed would be a bit bigger than the calamondin seed too. Your pic looks like it has one seedling next to what looks like a seed coat.

I have lemon and calamondin seedlings, about 3in tall. The calamondin has somewhat narrower elongated leaf shape than the lemon.