There are several types of meat thermometers. You'll find the two most basic styles, the bimetallic and bulb thermometers, at most grocery stores. These are inexpensive options which are easy to find, but they can take much longer to give a temperature read-out and aren't as accurate as other options. Also, their glass parts can easily break.
Digital instant-read thermometers provide much more accurate results. There are two main types in this category:
A digital instant-read thermometer gives you an (almost) instant readout, and it's easy to use.
A digital probe thermometer, which connects the probe that you insert into the meat with a separate device that contains a temperature readout and customizable alarm settings, is great for roasting or smoking larger cuts of meat for long hours.
CALIBRATE YOUR THERMOMETER
To quickly test if your thermometer is accurate, dip the tip into a bowl of ice water. It should read 32°F or 0°C, the temperature that water freezes at. Many digital thermometers have a reset button or re-calibrate button, so if the temperature is off, you can likely fix it—just follow the manufacturer's instructions.
PLACE THE THERMOMETER CORRECTLY
For the most accurate reading, place the thermometer into the thickest portion of meat, avoiding fat and bone. You're looking to find the lowest internal temperature—that's the most accurate temperature for the core of the meat. Most thermometers require you to insert the probe at least 1/2 inch into the meat, but if the meat is thicker than an inch, you'll probably want to go deeper than that to reach the very center.
The temperature should keep dropping as the probe goes into the deepest part of the meat—if you see the temperature starting to rise again, you've gone too far.
CHECK THE MEAT TEMPERATURE EARLY AND OFTEN
For a larger roast, start checking your meat about 30 minutes before you expect it to be done; for thinner, smaller cuts, start testing the meat 5 to 10 minutes ahead of time. To hit the right doneness, aim for the meat temperature given in your recipe, as well as food-safety charts.
It's important to remember that meat will continue cooking after it's removed from the heat—this is called carryover cooking. It's not much of a factor with smaller cuts of meat, like chicken pieces, steaks, and chops, but large, thick roasts of beef, lamb, veal, pork loin, or even large turkey breasts should be removed from the heat when they reach 5 degrees less than their desired doneness temperature. Give these larger cuts 5 to 10 minutes of resting time, and the temperature will rise up to perfect doneness and the juices will have plenty of time to redistribute into the meat.