Simplest thing you want to do, to differentiate, is to take a picture with very narrow F stop say 32 22 if your tele or macro lens has one or at least 16 that any lens has. Subject is a lighted uniform color; say sky. Even if you have low shutter speed, it doesn't matter.
The sensor dust, scratch will shine up at this point. If you see nothing, have a deep breath. It is dust over mirror or on pentaprisoms surface etc that does not interfere with your image as that is the line of sight for eye, and not the line of capture on sensor.
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S a s s a n .
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