There will probably be two controls - one to set the temperature and as long as the markings have not worn away you should be OK with that. The general cooking temperature is 180 degrees centigrade plus or minus a bit - hotter for roast potatoes and cooler for things you wish to cook gently.
You do not explain your dilemma and leave it for people to guess (tch!-tch!) and my guess is that you can't decipher the ridiculous icons - I have this trouble too.
Clearly you do not have access to the user manual - you might try to find one on-line.
There will be a wavy line to indicate the grill - with and without the fan.
There will be a fan on its own with no bar to indicate heating.
There will be a single bar at the bottom for gentle cooking and two bars top and bottom for a more fierce heat - with and without a fan symbol.
The company a-part-4-U will sell you a manual for about £4
It may help for you to discover the model number - it will be on a rating plate (sticker) hidden behind one of the doors.
You can buy an oven thermometer quite cheaply - less that a fiver.
I do not own a Diplomat oven - I just guess on your behalf.
Good Luck...
April 2016