See if an address is already a user

Im implementing the moralis linking methods but need to check if an address is already a user. How do i check this in javascript ?

Something like

Moralis.Web3.onAccountChanged(async function (_addr) {

if ( _addr not already a user){

Moralis.link(_addr)

}

}

You could call a cloud function (with your masterKey) that checks if a user/account exists.