How do I know if I have Quickbooks Plus plan or not?

You can verify QBO plan couple of different ways.
You can easily verify that directly from the Billing and Subscription tab.
From the homepage screen > Click on the Gear (⚙) icon at the top right corner > Your Company > Account and Settings > Billing and Subscription > QuickBooks Plan details.

Gear panel

Another easy way is to check the Gear (⚙) icon panel, look for Budgeting under Tools. If you don’t see any side-up-arrow, it means you’ve PLUS plan. On other plans (Essentials and Simple Start), Budgeting feature is not available and will see side-up-arrow.

Budgeting