How do I mark an estimate as closed?

One suggestion I found in the forums is to delete the estimate but I cannot becuase I get this message:

"One or more progress invoices have been created from this estimate. Deleting the estimate will cause each of the invoices to lose its association with the estimate. Do you want to delete the estimate anyway?"

And I assume that I should rather not delete it in fear of loosing the invoice. 

I have hundreds of estimates. How do I know which ones I still need to convert into an invoice?