Dear Mehmet Kösem, the best methods are those named real-time monitoring reaction progress, mainly gas chromatography and infrared spectroscopy. An old method is the iodometry which quantify the amount C=C. Details are presented in polymerization textbooks. My Regards

Dear Abdelkader BOUAZIZ, thank you so much for your answer.

Styrene in workplace air can be determined by capillary column gas chromatography (GC) with a flame ionization detector (FID). The sample is adsorbed on charcoal and desorbed with carbon disulfide. This method (NIOSH Method 1501) has an estimated limit of detection of 0.001–0.01 mg per sample (Eller, 1984).

Dear Chuck A Arize, thank you so much for your answer.