Usually a bleach stain is a chemical reaction that has created a colour loss.  It may not be bleach but it has created a loss of colour.  Usual rule of thumb if spot is lighter than there  is a loss of color, if spot is darker it is usually a soiled area.  There is a difference between a spot and a stain.  A spot can usually be removed with normal cleaning proceedures.  a stain may require using a chemical agent like a stripper etc. to reverse the colour or simply spot dyeing the carpet.  Hope this helps.  Just remember the spot or stain belongs to the customer.  The hole belongs to you!!

Actually how do you differentiate a bleach stail from a discoloration or a similar stain supposing the owner does not remember or is lying ?

Ted has a good dying company.... but it all depends on the type of carpet.  I know nylon can be and wool I think .....but olefin , polyester and a few others can't...

I know guys who charge from 150 up per sq ft... it's some great coin I tell you