When you notice darker spots in the lighter areas of you image.
You can test it by shooting something light at a small aperture.
I use an ExpoDisc over the lens, but you can put a white sheet of paper on the lens, or point the camera at a piece of blank paper, wall or sky (w/o birds), stop the aperture down and take a shot.
Shutter speed won't matter as the spots are connected to the cover over the sensor.
Found a video that's close to my procedure HERE.
Contrary to the video, I overexpose by a stop.
No need to download and look on your computer screen- just zoom in and scroll around the image.
You won't see them at wider apertures unless they are really bad.

I usually shoot pretty wide open, so I only notice when doing close-up product shots (at small apertures), like dental instruments on a white background.
I'm usually shocked at how dirty the sensor is.

Dust in the lens never presents as spots and if you're using a DSLR, dust on the mirror or in the viewfinder NEVER shows up in the image.

When you notice darker spots in the lighter areas of you image.
You can test it by shooting something light at a small aperture.
I use an ExpoDisc over the lens, but you can put a white sheet of paper on the lens, or point the camera at a piece of blank paper, wall or sky (w/o birds), stop the aperture down and take a shot.
Shutter speed won't matter as the spots are connected to the cover over the sensor.
Found a video that's close to my procedure HERE.
Contrary to the video, I overexpose by a stop.
No need to download and look on your computer screen- just zoom in and scroll around the image.
You won't see them at wider apertures unless they are really bad.

I usually shoot pretty wide open, so I only notice when doing close-up product shots (at small apertures), like dental instruments on a white background.
I'm usually shocked at how dirty the sensor is.

Dust in the lens never presents as spots and if you're using a DSLR, dust on the mirror or in the viewfinder NEVER shows up in the image.