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I try to have very good flats because I'm using my images for photometry. I take flats through each filter separately as there is a little bit of difference in each. It is very hard to get perfect flats.
You can check the flats by applying them to an image that is basically just a star field. ie no nebula and then stretching the histogram to exagerate the background. It should be perfectly even across the entire field.
The image I have attached is a V filter image of the nova KT Eri.
It is stretched to show only values between 260 and 330 in a 16 bit image (ie 0 to 65000)
It was a 30 sec binned image 2x2 from my ST10XME and I have shrunk it 50% to display here.
It is not perfect after flat fielding. There is still a little bit of uneven illumination in the left corners but it shows the effect of the flat well.
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