What is the best wavelength use in testing fiber optic cable?

Hi! I just want to ask which wavelength is best used in testing a fiber optic cable? For example, for a singlemode fiber, with wavelength of 1550nm, is it best to use 1550nm or is it also good to use other wavelength like 1310nm for testing?

Fibre optic cable should be tested at the wavelength at which it is intended to be used.  This varies depending on whether the cable is single mode (usually used for longer distances) or multimode) usually used for shorter runs around buildings for example).  Plastic optical fibre, used for really short distances, is different again.
I found a very good article on this here which covers which wavelengths are used.The FOA (from which the above is taken) has a wealth of information on all aspects of fibre optics, there is an index of the topics here.With regards to the last part of your question, single mode fibre is sometimes used for WDM (Wavelength Division Multiplexing) where a number of streams (light beams) are sent down the fibre at different wavelengths - much as you might use channels at different frequencies in a radio system. FOA's site has many articles on it - have a look around, there are a couple of links in my first link above.  When using a fibre for WDM you'd want to characterise and test it at all the wavelengths that will be used.You could spend hours on this site...there really is a lot of information.