The pizza at Costco is not good. That being said if you want to try it, you can almost certainly do it without a membership if there are only a couple of you. If there is a big group, the advice below probably won't work.

You need a membership card to get into the club or to buy regular merchandise. However, once you are in, you don't need a card to buy items from the food court.

Enter through the exit. Tell the person checking receipts at the exit that you want to go to the membership desk. Wait in line for a few seconds and tell the people at the membership desk that you just moved there and are deciding if you want to join. Ask if you can look around the store. They'll say yes. Walk a few dozen more meters into the store and you'll see the food court. There are giant pictures of the food up on the wall and tables. You can't miss it. From there, you can order the pizza without a membership card.