How to tell kittens appart

So I took in 2 rescue kittens. In the past when I had young rescue kittens they looked differnt enough to easily tell appart. The issue with these two is they are both orange tabbies and look ALOT a like. One is bigger than the other, but the little guy is catching up now that they are with me. I don't want to use yarn because it seems unsafe. They are about 3 - 4 weeks old and I'm just not sure what to do to tell them apart when it comes time for them to be adopted. I would also like to name them, but don't feel like there is a point in giving them names until I can tell who is who. I'm wondering if other people have things they use?

I have used Welping collars (bought from Amazon) and Spiral Stretchy Keychains (Amazon or Wal-Mart.)

If using Welping collars, make sure they are snug, but not too tight. Need to be able to put 1 finger in and check the tightness daily.

I used different colored yarn with my last litter.  Agree with @CatPack  - make sure they are tight enough to stay on, but at least a finger's amount of room.  And yes, check daily and replace as needed.  The kits will grow quickly, and need larger replacements regularly.

Since they are not purebred and they will be adopted out anyhow, I don't think it's important to be able to tell them apart, at least not at this tender age.  Just give them a couple of brotherly names (like Ben & Jerry, for instance) and don't worry.  Since you will be keeping their medical records and they will likely be identical as to when they are wormed, when they are vaccinated, etc., it really doesn't matter.  I had two bottle babies like that, both orange tabbies.  When they were 3-4 weeks of age, I just called them "Loud" and "Louder" because that was their only difference.  They were adopted out but unfortunately returned when their family was transferred by the company he worked for, and forced to sell their home and live in company housing which only allowed ONE pet.  They had a German Shepherd, age 10, that the family had gotten as a puppy, so the two boys were returned to me (I always offer to take back any cats I adopt out, for any reason, at any time).  I don't even remember what they had named them, but they are now named Mango and Chutney.  Why?  Because Mango is sweet and Chutney is "spicy" -- I dont even remember who was loud and who was louder, but I can tell them apart now that they are a year old.