You do not need to open cPanel to see where you stand.
WHERE
Services > My Services > click the service. The page shows disk usage and bandwidth against your plan limits, along with your package details.
WHAT TO WATCH
Disk usage approaching the limit. A full account stops accepting uploads and stops delivering email, which produces confusing symptoms that have nothing obviously to do with disk space. Bandwidth approaching the monthly allowance before month end.
FOR MORE DETAIL
cPanel gives the full picture: Disk Usage breaks space down by folder, the statistics panel shows the inode count, and Metrics > Resource Usage shows CPU, memory and entry processes over time.
Inodes are worth knowing about. Many accounts hit the file-count limit before running out of space, particularly with several WordPress sites and busy mailboxes.
WHEN USAGE IS HIGH
Clean up before upgrading: old backup archives on the server, cache folders, mailbox trash, abandoned installations and staging copies. Frequently that resolves it entirely.
If you remain close to a limit after cleaning, the account has genuinely grown. Open a ticket and we will look at the figures with you.