If you look at the Cost Analysis section of the Microsoft Cost Management service it will show the virtual machines that have a compute cost associated with them. These are PAYG costs.
Depending on your Azure subscription type, you may be able to see costs associated with your RIs here also but essentially you need to track your costs here. If you have RIs that cover all of your Virtual Machines fully then there should be no VM compute costs showing here. Anything that is not covered by RIs will appear here as PAYG compute costs.
https://learn.microsoft.com/en-us/azure/cost-management-billing/costs/quick-acm-cost-analysis