The test is designed to show up any incompatibility of the hair with the materials to be used, to allow the operator to correctly choose the required type and strength of product and to gauge the shortest safe time and lowest temperature needed for satisfactory processing.  

A suitability test involves full length of one or several strands separated out from near the nape of the neck, isolated from the bulk of hair.  
Alternatively a mesh or meshes are cut off from near scalp level and laid out upon an inert surface.  
Having identified the hair condition and texture and the maximum processing time allowed by the product manufacturer's Timing Guide (the Instructions) the chosen chemicals are then applied to the entire mesh length of hair, if in situ keeping at least 1cm from the scalp but otherwise then processed strictly according to the product manufacturer's instructions or guidelines, combing and smoothing with the back of a wide-toothed comb.  
The chemicals must then be removed by thorough washing and 'neutralisation' in the order stated by the manufacturer.  
If using a hydroxide relaxer this neutraliser will usually have a colour-signalling component of shampoo supplied with the product that should in use indicate correct removal of alkalinity of that relaxer.   If using a thioglycolate straightener there is usually no such colour signal component.
Careful and thorough neutralisation and complete chemical removal is an important part of this test.  
The test strand must be left to dry naturally without heat, then checked for straightness and any sign of breakage.   Such a test takes time and should not be done on the actual treatment date but a day or so earlier, to allow for investigation of any issues that might arise For the test only , the chemical product intended to be used on the hair whether thioglycollate straightener or hydroxide relaxer should include and cover the regrowth and overlap onto any previously chemically-treated part of test-hairshaft (if any).  
This test length should be processed for at least the normal processing time of the product for that hair type, checking throughout processing at 3-minute intervals.   It is thus a test for chemical compatibility with any possible previous treatment but will of course involve the necessary disadvantage of over-processing at the overlap for these test purposes of that test-only strand.   Any change in colour, heat or tensile strength down the whole hairlength should be noted.  
The minimum time required to give the desired smoothness and straightness should also be noted.  
The chemicals are then washed off and the intended neutraliser applied and processed in the order given by the manufacturer.  
After all chemicals have been completely removed, the tested strand(s) must be checked again for tensile strength and any sign of discolouration or damage.  
Apart from an expected weakness at the overlap as purposely done for this test, only if there is no sign of hairshaft damage or breakage, discolouring or unexpected weakness should the proposed treatment be (later) carried out, using the timing parameter now found.

Note:   Since a client's body-chemistry may change over time and since one can never be sure with what a client has previously been treated, a Suitability Test should be made prior to each and every application of either a relaxer or a perm-type straightener, in order to ascertain the current wisdom of whether or not to proceed. But remember, when later treating the client's whole head of hair, any overlap of previously chemically treated hair must be strictly avoided , to eliminate the serious possibility of breakage.

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NB: Remember that a hydroxide 'relaxer' will always be incompatible with a thioglycollate 'straightener' sometimes called 'straight perm' or 'curly-perm' - and if the former relaxer is overlapped onto any of the latter previously straightener treated hair - or vice versa - hair breakage from the point of overlap will inevitably result. Always remember that these chemicals should never come into direct contact with the scalp, or forehead ears neck or other skin, since tissue burns will most likely result from such contact.

©    D Dane 2007