This script will allow you to check all user mailbox forwards and shared mailboxes forwards enabled in your tenant.
You will need to know how to save a mailbox script. A quick guide is to do the following:
1. Copy the script below
Get-Mailbox | select UserPrincipalName,ForwardingSmtpAddress,DeliverToMailboxAndForward | Export-CSV UserMailboxForwards.csv -NoTypeInformation
2. Paste it into the notepad file and when saving, copy the below.
2a. You can then run them via Powershell. I normally CD to the folder in question and then to run a script, you will need to type the following .\ in front of the name to order Powershell to run the script.
It will then save the results in a .csv.
The two scripts you will want to run are:
Get-Mailbox | select UserPrincipalName,ForwardingSmtpAddress,DeliverToMailboxAndForward | Export-CSV UserMailboxForwards.csv -NoTypeInformation
Get-mailbox | where {$_.forwardingaddress -ne $null}| select UserPrincipalName,ForwardingAddress,ForwardingSmtpAddress,DeliverToMailboxAndForward | Export-CSV SharedMailboxForwarding.csv -NoTypeInformation