Though SVKitchen and other recipe sources provide cooking times, sometimes you want to figure out a new sous vide recipe for yourself.
You can arrive at the correct temperature and cooking time through extensive trial and error or you can use technology to tell you when your food is done.
We use an Extech 421502 Dual Input Thermometer with a submersible Miniature Needle Probe THS-113-181. The thermometer is available from multiple vendors. The probe is sold by Thermoworks, who also make the speedy and accurate Thermapen.
The thermometer is rugged and water resistant. Best of all, you can set an alarm so when the probe reaches the target temperature, it lets you know.
After you have evacuated air from your cooking bag and sealed it, apply an inch of weather seal foam to the bag and stick the probe through the foam into the food. The foam helps to maintain the seal.
We learned about this equipment combination from a post by Nathan Myhrvold in a long-lasting sous vide discussion on eGullet.
If you don’t want to invest another piece of cooking gear, you can make do with a standard probe thermometer to check internal food temperature. Most, however, are not sufficiently precise for this use.