It is possible that you have a general low level of leakage, and the slight extra from the oven element pushes the level above tripping point.
Most common causes of leakage are watery appliances like kettles, washing machines, tea urns and immersion heaters. There is also a bit from computers due to the way their power supplies are built. Large heating elements like ovens are also known for it. Sometimes the currents travel round odd paths to cause a trip
Apart from unplugging suspect appliances, I don't know an amateur way to test for it. If you know any electricians or people who do PATesting they could track it down, the tester is some hundreds of pounds but might possibly be available to hire. A clamp meter round the earth wire will also detect currents but to trip an RCD the current is very low and an inexpensive meter would probably not be accurate enough.
An RCD testing instrument is also fairly expensive. A lot of people just plug it in but you ought to test the RCD in isolation from the circuits as well.
Start by having a good look round for any signs of damaged cables or accessories, or water leaks round electrical parts (central heating pumps and boilers are a common source). You can also try unplugging (not just turning of) half your electrical items for a day and seeing which ones seem to affect the tripping. If you get one of those RCD adaptors as used for lawn mowers, try plugging in the washing machine and kettle through it (though this will only work if you have a single major source of leakage).
Meantime ask around friends and neighbours for a local electrician they recommend. If engaging one, always ask first if he is a member of a self-certification scheme; which one; how long he as been a member; and what is the full trading name he is registered under (this enables you to check).
