Richard
You can't give a price that's fair to you or the prospect until you know exactly what's involved. A good example is dye bleed. More often than not, a dye can be stabilised by a pre-treatment, in tank rinse or post clean treatment. So how can you cost it fairly if you don't know it will happen?
I have to put my hands up and plead guilty to not always carrying out all tests every job, but as Chris says, that's down to experience. (I had to admit that as Chris B and I have worked together

) Certainly, If I had any doubt with a new customer and an unknown carpet, I would always test.
Something I am going to repeat yet again, because I think it's very important, is that the final examination occurs when you carry out your Dry Extraction (vacuuming) as during this part of the process you will/should be looking at every sq. cm of the carpet quite carefully. Once you have passed this stage and are commited to water/chemical, there's no going back, so any damage will be down to you.
Safe and happy cleaning

Ken