If you want to know if your bytes are linked, then say hi to the transition bot, and it will tell you your linked Byteball addresses with their balances.

If you want to know how many bytes are linked in total, well that would only be relevant to know how many blackbytes will be distributed, knowing that the non-distributed blackbytes will serve for development, and that all bytes in circulation (100 000 GB) will receive bytes whether they are on linked Byteball addresses or not.