Check Salesforce organization limits and usage using Workbench

1. Ensure you are logged into the organization where you want to verify your limits.
2. Navigate to: https://workbench.developerforce.com/login.php
3. Accept any oauth prompts to complete authentication
4. On the ‘Jump to’ picklist select REST Explorer.
5. Click Select.
6. From the options presented select: /services/data/vXX.0/limits
7. Click Execute.
8. Select the SingleEmail area to view the daily maximum and remaining calls.

Reference: https://help.salesforce.com/articleView?id=000331939&type=1&mode=1