How do I test the quality of hair extensions

That depends if you were looking for human hair extensions or synthetic hair extensions. To be honest both their extensions can look nice if done by a talented stylist. You should be careful because a lot of places will try and sell synthetic hair extensions as human hair extensions to naive buyers. I like to look at the extensions carefully. if the extensions appear to be all solid color and the hair all the same texture usually I assume they are synthetic. Upon looking at extensions that are marked human I check the hair for minor differences. Like a bit of a different texture and often you may find a gray one or two. If you do buy hair extensions as soon as you get them home I would clip a tiny piece off. I didn't burn a piece of the hair synthetic hair extensions curl up and smell like plastic when burned. Human hair extensions and Yaki turn to ash when burned and smell like burning hair. To test this take some hair out of your brushes or combs, burn it and compare the smell.