How to check the inventory for RI in Cost Explorer?

Please find the response,

For the first query, you can go the AWS Savings Plan console and add custom dates from the day the SPs are purchased and filter it using the savings plan type along with that you can go to the Savings Plans utilisation breakdown then change settings only to view Savings Plan ID, Net savings to have the clarity. (Utilisation will be selected by default). Also, you can download excel for your reference to show it to the customer.

Coming to second query, In Reservation inventory you will have the following:

• Amazon Elastic Compute Cloud - Compute
• Amazon Relational Database Service
• Amazon Redshift Click on Net Savings of any of these which you want a inventory list for and it will redirect you to the Reservation Utilization report page then you can download the excel the sheet from there or you can scroll to the bottom where you can filter all the RI subscription IDs.

PS: you will not be able to check the forecasting information for the Savings Plan using Cost Explorer

I hope this helps.