If you search an Office 365 user with Get-AzureADUser or Get-MsolUser, you get details about the account type with the attribute UserType. The value can be:

• Member: the user is part of the Azure AD tenant
• Guest: the user is a guest, for example to access to Microsoft Teams or SharePoint site

According to this Microsoft blog, the UserType attribute was first introduced the 31st August 2014, so every user created before this date has the UserType attribute empty.