Hi, I would like to find out if an invoice has been posted in QB. How can I search it using the invoice number?

Hey en2, 

Thanks for your post and welcome to the QuickBooks Online Community!

You can use the advanced search feature by going to the magnifying glass on the top toolbar > advanced search.

Here, you can set the fields to search for 'Invoice' and 'Invoice No.', then search the specific number to locate the invoice.