Yes, you can start trying to sex them.
I would probably check again every week or so until I'd gotten the same results multiple times for each one, and then check maybe every month or so until they are fully grown, just in case.
https://www.backyardchickens.com/threads/rabbits-how-to-sex-your-bunnies.140990/
https://www.backyardchickens.com/threads/rabbit-sexing.1292081/
Here's one thread that tells/shows how, and another with more photo examples of what you might see.
The basic idea: hold the bunny in the right position, push gently on a particular clump of fur, and the parts you need to see will become visible.