With the lens off, look at the mirror and the prism glass. (It is inside the mirror compartment, on the top. Looks like a glass square)
Use a puffer brush gently and a cleaning tissue and lens cleaning fluid sparingly. Mirros clean easy, but you still need to becareful.
90% of the time what you are seeing is on that glass square and does not effect photos. (Mirror pops up and covers this when a photo is taken.)
To test for dirt on the sensor:
Take a photo of blue sky, no clouds, blue wall, or such, on ISO 100, finest JPEG, F16 or F22 is better, using auto settings. In you photo software open the image and view at 200% and increase contrast until spots just appear. Look closely to see if it is dirt or digital noise.
Generally noise is normal and looks like even grains of sand, washing over darker parts of the image. (Properly exposed blue sky should be even and have no dark parts so only dirt appears)
Clean your sensor according to whatever your manufacture’s says and repeat to test results.
If you are unhappy with these results, try this:
Set the camera to manual, ISO 100, F16/F22 @ 1 second. Put the lens cap on and take a picture. (It will be dark on purpose)
Load it into your photo viewing software and adjust the brightness or contrast slightly and watch for noise, specks and hairs to appear.
Clean your sensor according to whatever your manufacture’s says and repeat the cap on photo.
Compare the two photos and you should see a difference.
Some cameras have auto self cleaning modes, if you got it, use it.
NEVER TOUCH OR CLEAN THE SENOR WITHOUT READING THE OWNERS MANUAL FIRST.
If you do, and you ruin it, you will leave a fingerprint and the manufacture will use that to void the warrantee. Follow the manual and you should be fine, or if you are over your head, find a local camera shop that cleans digital cameras to do it for you.
NRA Life Member
The scariest words in the English language: I'm from the government and I'm here to help! President Ronald Reagan