How to check the current provider of a domain's email
You can use MXToolbox to check the location of a current domain. To do so, go to https://mxtoolbox.com/ and enter the customer's domain name, then click MX Lookup.
If your customer is already on Office 365, their MX record will return (domain).mail.protection.outlook.com, and MX toolbox will indicate Office 365 as the email service provider.
MX Toolbox recognises several of the main email providers, however may not be able to detect them all.