As VictorIvanidze-0112 said, you could find the add-ins list from registry.
Here is a script which could help you read the registry (Run it on the computer that you want to check):
$searchScopes = "HKCU:\SOFTWARE\Microsoft\Office\Outlook\Addins"
$searchScopes | % {Get-ChildItem -Path $_ | % {Get-ItemProperty -Path $_.PSPath} | Select-Object @{n="Name";e={Split-Path $_.PSPath -leaf}},FriendlyName,Description} | Sort-Object -Unique -Property name
If you want to check for all users, you could try to run this script with a GPO.
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