On zkSync Era, fan-in means many accounts paying one address, and fan-out means one address paying many. The address format shows neither pattern, so you read it from the activity itself.
Counting the edges
Count the distinct senders that feed one account, then count the receivers it pays onward. Short gaps between the two sides matter most. The free check gives the activity count as a first signal of how busy the account is.
Smart accounts change the picture
Native account abstraction means a smart account can act for a user, and a paymaster can pay the fee. Look at who sent value, not only who paid gas.