If you want an easy way to get a list of all the mailboxes that are currently hidden from the GAL, you can use the following cmdlet:
Get-Mailbox -ResultSize Unlimited -Filter {HiddenFromAddressListsEnabled -eq $True}
You can also export all their information to a CSV file:
Get-Mailbox -ResultSize Unlimited -Filter {HiddenFromAddressListsEnabled -eq $True} | Export-CSV D:\HiddenMbxs.csv