Different lengths need to be looked at in order to be able to work out what the hair's condition is. For example, many people can have dry ends (from chemical or heat damage) and oily roots, so depending on the length look at the roots as one diagnosis and the ends as another. If the client has very long hair, you can also consider the middle sections as they could also be different. Below is a list of different hair conditions and a guide to the giveaway signs for easy diagnosis.
Normally always at the root of the hair and very rarely at the ends. Oil through the lengths of the hair is eliminated by use of colour and heated appliances, which are both very common these days. Oily lengths can often look a little glossy, as oil makes hair look slick. Oil also makes hair limp, and you can struggle to get it to do anything. It is often thinner hair that has problems with oil - thicker hair trends to be more dry and brittle as not enough oil is created to make it supple.
This is what everyone wants. Shiny, glossy, bouncy hair, which feels silky, and shines constantly. Not oily, no dryness - just perfect.
There are two kinds of dryness, and many people confuse the two. One is dryness from lack of moisture, or oil, and the other type is hair that often feels dry from the damage that has been caused inside the hair's structure, normally called dry/damaged or sensitized hair. This sort of hair is also dry to the touch, but is often fragile, too, and breaks quite easily. These are tests which can identify this better.
If the hair structure is damaged, the hair is weakened. This makes the hair look dry, but if you use a reconstructing treatment (which is for strengthening hair) the dryness will also disappear, even though reconstructs are often in liquid form where conditioners for dryness come in cream form.
To recap, dry hair is caused by lack of moisture and external damage, and dry sensitized hair is caused by internal damage.
Combination hair is a mixture of the above, e.g. oily roots and dry ends; dry hair with dry sensitized ends; normal roots with dry ends, etc.