A Good Hairstylist must look, smell, talk, listen, feel and think!

I had a client come in for a relaxer service. I felt that I could smell "Sulfur 8 hairdressing" in her hair so I asked the client "are you using any sulfur based products?" She said "NO." I then reworded my question. "Have you ever used Sulfur 8 hairdressing before?" Then the client said "yes, about six days ago." So then I had to explain how Sulfur based products and relaxer services do not go well together. I declined her the relaxer service, and we had to shampoo and press out the roots. She still left with a great hairstyle and scheduled for another week for a relaxer service.
This was a perfect example where hairstylists need to know their "product smells," use their common sense, talk and listen to the client before peforming any chemical service.
Another example where hairstylist need to be cautious is with "permanent haircoloring." The clients sometimes think that because they "re-colored" their hair back to its original "dark" color that it is not color-treated. Once hair has been permanently altered by lightener or permanent haircolor, it is still considered "color-treated" hair no matter what rinse you put on top of it. A mild relaxer strength is the only strength of relaxer recommended for color-treated hair. Sometimes a hairstylist has to feel for excessive dryness in the hair ends in order to get a feel for whether the client has been experimenting with hair color. Again, using your sense of feel comes in handy.
Also, if the client is over age 40, and you do not see not one single strand of gray hair, then you can safely assume they are coloring or rinsing their hair. If it does not feel dry to touch then it may be a semi-permanent color, but if you feel any roughness, then it is probably a demi-permanent or a permanent color or a henna.
Henna and relaxers are non-compatible. I was told there are some body art grade hennas that are safe to use with a relaxer, but I would not risk it!

November 2012 update:  I just had a new client to come in wanting her sideburn areas relaxed because they were too frizzy.  I explained to her that frizz could come from damage to the hair.  If that is the case, relaxing damaged hair is not going to resolve the frizz.  We opted to cut the sideburn areas down without the chemical processing.  I also introduced her to the Silk Elements edging gel.  Her loose curl pattern molded down nicely.  The haircut was the perfect solution.  It can still be relaxed at a later date if she so chooses.  She is trying to stick with the "press and curl" procedure for her entire head.  If I was really focussed on making the money off of the relaxer service, then I would have relaxed her sideburns, and then I may have still cut it down.  However, for me, I know that relaxers can be more damaging to the scalp, and I felt that she already had some reservations about full relaxer services.  So I decided that the extra revenues that I could earn from a relaxer service was not necessarily the best solution to the frizziness that the client was wanting to eliminate.  The client really appreciated me giving her my professional opinion as well as presenting her with reasonable options and giving her time during our consultation to make an informed decision as to her next step. Mission accomplished.