Hi,
You can see if it is OS disk by looking at the OS type column for the disk, on Overview tab this shows as "Operating system:" Additionally OS disks created via the portal during manual VM creation will normally have OsDisk in their name.
You may view Disks in portal using below link:
https://portal.azure.com/#view/HubsExtension/BrowseResource/resourceType/Microsoft.Compute%2Fdisks
Click Manage view -- Edit columns and add the OS type column.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP