I own a Sony a6500 and primarily use it for filmmaking. I was trying to clean the mirrorless sensor and used a rocket air blower to get dust out of the sensor but accidentally knocked it against the sensor. I can't see any physical damage or scratches but not sure if I'm damaged the sensor internally. How can I determine if I've damaged the sensor?
The sensor isn't directly exposed - it's got a few filters on top of it, such as an anti-aliasing filter. Your tap with the blower probably did nothing to the sensor. (For what it's worth, I used to clean my 20D with a swab and methanol - so, made direct contact with it all the time).
But, power it up and make sure the camera isn't throwing an error code at you and then take some test photos.
I would make a series of test shots.
Out of focus test shoots
You will need, a tripod, a decent computer monitor and a graphics program, it can simply be MS Paint.
Put your camera in front of your computer monitor about 15 cm from it.
Set your lens at manual focus and focus to infinity.
Put your camera at the lowest ISO setting, let us say 100.
Set the aperture to a low aperture, for example f22.
Make some tests using different exposures and different targets, for example, pure white, pure red, pure blue and pure green. Some more overexposed and some underexposed.
Normal focused shoots
Look for a nice repeatable pattern, like a closeup of a piece cloth of Tartan (the Scottish pattern)
Move pan and tilt a bit the camera.
Compare your tests shoots
On your computer at 100% or 200% and see if you find anything. If you see something you probably want to check if it is not a dust speck.
If it does not seem to go away always see if you need to take it to a service center.
When we "clean a sensor" we are really cleaning the front of the filter stack that sits in front of the sensor. For different camera makers and systems the exact thickness varies, but is usually in the neighborhood of 1.5-2 millimeters. So "sensor" dust and any scratches or other damage to the "sensor" is usually a couple of millimeters in front of the surface of the actual sensor.
When the front of the filter stack has dust or scratches on it, we don't actually see the dust or the scratch in images taken with the sensor. What we usually see are the shadows cast by that dust or scratch. Sometimes in the case of a scratch we will see an area of increased brightness as the shape of the scratch on the surface of the filter stack focuses more light on a smaller area.
In either case, the best way to detect such things is to use light as collimated as we can. Collimated light is light in which all of the rays are parallel to each other. By closing the lenses aperture to a narrow position (high f-number), the aperture blades block most of the non-collimated light entering the lens from reaching the sensor.
Because of the much shorter distance between the lens and sensor than is typical between the lens and subject, the distances between the front of the filter stack and the sensor are much more critical than the distances between distant subjects. How blurry an object appears when it is a set distance away from the point of focus varies at different apertures. There is also a varying "depth of field" in front of the sensor for different apertures.
As the aperture is closed down the amount of light coming from the edges of the lens and striking the sensor at an angle is reduced and only the light coming from closer to straight on is allowed to reach the sensor. When a wider aperture is used some of the light rays coming from wider angles manage to strike the pixels directly behind the dust spots. When a more narrow aperture is used the dust blocks a much higher percentage of the light from falling on the sensor directly behind it since most of the light is coming from straighter in front of the sensor.
The wider angled light rays at large apertures also effectively spread the shadow of the dust spots over a greater number of pixels. This reduces the contrast between the shaded and non-shaded areas until at some point the shadow is so spread out that the difference is no longer perceived. Much the same thing happens in reverse when you point your camera at a bright star and defocus the lens until the light from the star is so spread out that you can no longer see it.