The easiest place to check this is in the Vendor Center.  Go to the vendor and select All Transactions in the Show box and All in the Dates box. If you don't see these options, click on the bopx with an arrow pointing left at the top right of the vendor screen.  If there are a lot of transactions, you might want to sort by amount by clicking on the Amount column header.  If you see a down arrow, it means you're sorting low to high.  If you click the column header again, it'll reverse the order.

If a bill has been paid, you'll see an entry for a Bill and a Bill Pmt-Check or Bill Pmt-CCard (depending on how you paid the bill).  If you only see a Bill, it means a bill was entered and hasn't been paid yet. 

If you see a Check or Credit Card Charge, it means you paid the vendor without entering a bill first.  This is okay as long as you are certain you didn't enter a bill.  What you don't want to see is a Bill and a Check or Credit Card Charge for the same amount.  This usually means that you entered your expenses twice, once on the bill and once again on the check or credit card charge. 

Remember, if you've entered a bil, you always want to use Vendors > Pay Bills to pay it.  Never skip directly to Banking > Write Checks or Enter Credit Card Charges.