Most yards know almost exactly what kind of cars they have in stock, and if they don't have it, you've saved a trip. And keep calling. Know what you're looking for. Have on hand the car make, model, year of manufacture and serial number. Be very specific about the part that's needed, and check it to see if there's a part number stamped on it.
Some parts fit all.
Yards have a printed or computerized data base that cross-matches parts. So, if the yard doesn't have a 1987 Nissan Sentra, it might have a 1984 Subaru with the same part you need. Wear covered shoes. While some yards will allow customers with slippers to enter, it's never a good idea to leave your feet unprotected. Some yards will remove the part for you, but others will allow you to remove parts yourself, and you might save a few dollars that way.