Customer statements are used to show customers a summary of their invoices, payments, credits, and balances. You can create 3 different types of customer statements in QuickBooks Online, depending on your needs. I can see why you'd like to review a preview of your statement before sending it to your customers. I'd be happy to explain how to do this.
To generate a statement, click the + New icon, then click Statement. You'll see a list of statements, and you can check off the one you'd like to preview/send. Once selected, click Print or Preview at the bottom of the page. This will generate a preview of what the statement looks like for you to review before sending it. This preview will show you all the details, as well as the total balance.