There is two methods for testing for polyester in a fabric, the most readily available is a burn test. Polyester will melt, fuses and shrinks away from flame. It smells like chemicals when it burns. Cotton will burn yellow and turns to ash.
The second method is by chemically dissolving the fabric and from the weight removed it is possible to determine the exact composition of polyester material. This requires a number of lab grade equipment and access to Phenol.
To determine the composition of different fibres in a blended fabric we have to apply chemical test. Because there may consist of both thermoplastic and non-thermoplastic fibres. So we cannot different them by physical test such as burning etc. But in chemical test if it dissolves in a specific chemical we can identify that fibre portion of fabric. Now if we weight the sample before dissolving in chemical and after dissolving in chemical we can easily find out the composition of them in the fabric sample. At last we will apply chemical for the rest fibres to identify it. Suppose the total weight of the fabric sample = a and after dissolving one portion we find the rest weight = b.