What age are the kittens? If they are around 6-7 weeks old, you could use a small break away collar. If they are younger, and haven't had their first vaccinations, I would recommend waiting for others to be around them until the first set of vaccinations.
Also, if they are younger, and to tell them apart for weighing reasons, you can try nail polish under the arm, (the Momma may still get to it and clean it off though.)
I am sure others will have suggestions, I use Sharpie markers, but they wouldn't show up on an all black kitten.
I would recommend keeping them with Mom until 11-12 weeks of age, after 2-3 sets of vaccinations and spay or neuter, before homing.
Good luck finding purr-fect furever homes.