Tonight with your 5 to 8 year old
The prompt
“What's something about you that's different from your brother or sister, in a good way?”
What to listen for
If they name something quickly and proudly, "I'm the funny one" or "I draw better", they have a clear, comfortable sense of their own spot in the family, separate from comparison.
If they hesitate or compare themselves unfavorably, they might be feeling a little overshadowed, and could use you naming something specific and true about them without mentioning the sibling at all.
If they say "I don't know," you can offer a gentle prompt like "well, I know one thing that's just you..." and let them react.
What this tells you
How easily they can describe their own unique shape in the family shows how solid their sense of being their own person is right now.