You can view your Reserved Instance coverage to look for anomalies or changes in coverage. For compute resources you should purchase Savings Plans instead of Reserved Instances.
In Cost Explorer, click on Saved reports on the left:
Click on RI Coverage:
You will see the default RI Coverage report. It is for the Last 3 Months, and is for the instances within the EC2 service:
Scroll down below the graph and you can see a summary of the costs and usage. Note that depending on the instance type and size, the On-Demand costs will be different per hour:
To help focus where you need to, click on the down arrow next to ON-DEMAND COST to sort by costs descending. This will put the highest on-demand costs at the top, which is where you should focus your RI purchases:
You have now viewed your RI coverage, and have insight on where to increase your coverage.