I would firstly suggest you get yourself on a good training course, using the wrong chemical on a stain can often do more harm than good.
Secondly never make any guarantees that you will get a stain out, it often looks like a simple job, but invariably turns into a nightmare where you are rarely the first person that's "had a go" at it.
Thirdly whenever viewing a stain with a customer always start with a sharp intake of breath, followed by two or three "tuts" the the following paratext:
"Do you know what it is?"
They will generally reply with "Cat p*ss", "Tea/Coffee", "Dog was sick", "Red wine", etc
"Have you used anything on it?"
Again general reply is "Vanish", "1001"
Release your breath in a big sigh..........and say, "I'll have a look at it, but can't make any guarantees"Then you'll get "Is that bad, what should I have used?"
"Well, we only recommend this MS spotter"
Then sell them a spotter, or two!
If you get the stain out, you're gods gift to carpet cleaning, if you don't, they'll uderstand that it was their fault in the first place and you tried your best!!