another way to check for dust spots is the following :

a) create a duplicate layer of image and select it (background copy)
b) equalise this layer – this will show dust spots you may not know you had
c) select background layer but keep equalised version ‘on top’
d) use healing brush to remove spots – this will remove them from original layer
e) turn off / on equalised layer to check you have got all the spots
f) delete background copy NB – make sure you have this layer selected.

Very easy to do and will show up the faintest of spots.