How can I know when hot oil reaches a temperature of 350-375 degrees for frying?
You can know by using a kitchen thermometer, which is a common method. If you do not have a kitchen thermometer, you can stick the end of a wooden spoon into the hot oil. If some bubbles form around the wood and they float up, then your oil is hot enough to cook with. See http://www.ehow.com/how_5626660_tell-temperature-cooking-oil-thermometer.html.
Stick your finger in it....NO, please don't do that!! You need a kitchen thermometer to check the temperature of hot oil. I'm fairly certain a meat thermometer, the type you'd stick in a roast, will work just fine for oil too. :)
if its a restaurant there should be a little gadget, which measures oil temp and also lets you know if anythings burning.