1. Figure out which QuickBooks you're actually using. This matters because the two products don't share this concept at all. If you installed software on a computer, you're on Desktop. If you log in through a browser, you're on Online, and the steps below are different.
2. Desktop, fastest way: press F2. With your company file open, hit F2 (or Ctrl+1 on some keyboards) to open the Product Information window. License Number and Product Number sit right at the top, along with your file size and other diagnostics.
3. Desktop, alternative route: the Help menu. Go to Help > About QuickBooks [your edition and year]. The same License Number and Product Number appear at the top of the window that opens.
4. No access to the installed software? Check your purchase records. Your original confirmation email from Intuit lists both numbers, as does your Intuit account under past orders if you bought it online rather than in a box.
5. On QuickBooks Online instead? There's no license number to find. Go to the gear icon, then Account and Settings > Billing & subscription to see your plan and billing details. Your Company ID, the closest equivalent identifier, lives under Settings > Advanced > Company settings.