Sometimes you can tell by the presence or absence of a petiole wing. Grapefruits usually have winged petioles. Lemons, usually not. Almost looks like a Ponderosa lemon to me, but it could be many things. Do as John suggests.

Thank you for the comments. Crushing the leaf, it does smell a lot like a lemon instead of an orange. It also has huge thorns.

@Dave, based on your comment it does look a lot like a "Yuma Ponderosa", especially because it looks like it was the rootstock for a Kaffir lime variety; the Kaffir lime branch is still on the tree but the rootstock variety has taken off and next season might start dominating.

Sadly no trunk tag :( How do I know when ripe? This is the first year I am getting any fruits from the tree, so I am not really sure what color the fruit will turn (if it will turn color at all).

It will turn yellow or orange almost surely; and if it fall from the tree, you will know it is ripe. If you have several fruits, you might try one earlier and then a bit later. Trial and error. Oh and thanks for the info about Kaffir; I am searching for a rootstock that is compatible with Kaffir; so I guess I will write off Ponderosa.

Agree it looks like Ponderosa....their leaves have a really nice lemony smell and the leaves a fruit look consistent with Ponderosa. Could be wrong but the size of those fruits narrows down the possibilities quite a bit!

But you didn't mention before that it was likely a rootstock. So I doubt Ponderosa would ever be used as rootstock.

Could be Ponderosa, could also be Cuban Shaddock.

https://citrusvariety.ucr.edu/citrus/ponderosa.html

https://citrusvariety.ucr.edu/citrus/cuban_shaddock.html

https://citrusvariety.ucr.edu/citrus/yuma_ponderosa.html

Cuban and Yuma are used as rootstocks, and are closely related.

Usually larger fruited than true Ponderosa Lemon.

I had a Cuban shaddock that really produced fruit and grew very well very quickly. Flowers produced fruit-lets that didn't drop. It is the only citrus tree I have ever had that grew well and fruited even in adverse growing condition.