The only way to know is by using an oven thermometer. There are basically three types of thermometers out there for you to choose and use. I have and use two of them but have to admit that one doesn't get used much anymore.
|Oven Proof Thermometer|
The first type is your old standard meat thermometer which is oven proof. This is the one I don't use much because technology has improved to make it much easier to get a temperature. However, there is nothing wrong with this thermometer. To use this thermometer, you simply insert it into the thickest part of the meat and leave it while your dish bakes. Remember not to touch a bone with the thermometer as that will cause an incorrect reading. The thermometer usually has readings in both Fahrenheit and Celsius. Here in the US we use Fahrenheit in our recipes. I believe the rest of the world uses Celsius. The reason most people don't use this thermometer today is the convenience of the other two types. This thermometer requires you to either stick your head in the oven to read it or pull the dish out of the oven to check the temperature.
|Demote Oven Thermometer|
Our second type is the one I use when I'm looking for an internal temperature of a dish. It really works the same way as the thermometer above but it has a remote temperature reader that sits outside the oven. This remote reader has an oven proof cord that connects the probe in the meat with the remote reader. So you stick the probe into the meat just as you would with any other type of thermometer and set the reader on the counter next to the oven. One of the main reasons I like and use this thermometer is the bells and whistles that come with it. The best one is an alarm that you can set for the desired temperature. So if you are looking for a temperature of 165, you just set the device to 165 and an alarm will go off when the dish reaches that temp. I think they are pretty slick and help me so I don't forget to check the temperature when using the first or third type of thermometer.
Speaking of the third type, it is an instant read thermometer. You just stick it into the meat or item being tempted and it will give you an instant reading. This type of thermometer is better suited for checking temps of burgers and steaks than items in an oven. As with the first one, you either still have to stick your head in the oven or pull the item out.
I'd probably would be better off if I got rid of the first one and used just the second and third choices in thermometers. The choice is yours as to which one or ones will meet your personal needs. My favorite places do carry these items so check them out at Kohl's, Food Network and QVC.