Whenever I have birds of the same breed, I just spend some time observing them. After a while, you begin to notice subtle differences (one has a bigger comb, shorter beak, bigger earlobes, different face shape, etc.). It's hard when they are chicks, since they change every day, but once they start to get a little bigger it becomes a little easier. That being said, I know for a fact that I am the only one that can tell all my chickens apart! So while these differences feel obvious to me, they are often much more subtle than I realize. Sometimes it takes a while for me to notice something different, but once I do, I can't unsee it.
If you're still having trouble, you can try putting different colored leg bands on them. Just make sure they're not picking at them too much.
Hope this helped a little. Good luck!