There may not be a quick way to do this from Azure portal directly.
Look at this blog from Feb, 2018 about a PowerShell module created to tackle exactly this kind of scenarios by Azure Rapid Response team.
Use AzureRMAppGWCert to list certificate details of Azure App Gateway
This Module is Published at https://www.powershellgallery.com/packages/AzureRMAppGWCert
In order to install just open the powershell as Administrator and type:
Install-Module AzureRMAppGWCert
Import-Module AzureRMAppGWCert
Example: Listing all digital certificates associated with a single Application Gateway:
This Module will list all certificates associated with Application Gateway and generate an output like this:
PS C:\> Get-AzureRMAppGWCert -RG OfficeClient -AppGWName AppGateway
AppGWName : AppGateway
ListnerName : appGatewayHttpListener
Subject : CN=*.hepoca.com, O=Hepoca Armarios e Servicos Ltda - EPP, L=Taguatinga, S=Distrito Federal, C=BR
Issuer : CN=DigiCert SHA2 Secure Server CA, O=DigiCert Inc, C=US
SerialNumber : 0E99D5E2EBBE329CFE2DDE29C1D7D343
Thumbprint : 5FD6F2A7BC4BD095198AE55D1A0A76D46365C6B9
NotBefore : 3/13/2017 7:00:00 PM
NotAfter : 5/2/2018 7:00:00 AM
AppGWName : AppGateway
ListnerName : HTTPs8080
Subject : CN=*.hepoca.com, O=Hepoca Armarios e Servicos Ltda - EPP, L=Taguatinga, S=Distrito Federal, C=BR
Issuer : CN=DigiCert SHA2 Secure Server CA, O=DigiCert Inc, C=US
SerialNumber : 0E99D5E2EBBE329CFE2DDE29C1D7D343
Thumbprint : 5FD6F2A7BC4BD095198AE55D1A0A76D46365C6B9
NotBefore : 3/13/2017 7:00:00 PM
NotAfter : 5/2/2018 7:00:00 AM
AppGWName : AppGateway
HTTPSetting : appGatewayBackendHttpSettings
RuleName : rule1
BackendCertName : webjson-pub
Subject : [email protected], CN=webjson.arr.local, OU=Arr, O=ARR, L=Irving, S=TX, C=US
Issuer : [email protected], CN=webjson.arr.local, OU=Arr, O=ARR, L=Irving, S=TX, C=US
SerialNumber : 00B1722AB4D0FB8CAA
Thumbprint : 573C70769A40CF4D01769926A212009598462436
NotBefore : 11/28/2017 12:45:23 PM
NotAfter : 11/28/2018 12:45:23 PM