The vast majority of stocks or buffers will be mil-spec these days but if you are unsure you can measure the buffer tube if you have one or start with purchasing a mil-spec stock and then get a mil-spec buffer tube. The best way to measure a buffer tube type is by using a caliper or micrometer and measuring the diameter of the tube. A mil-spec tube should measure about 1.146”, and a commercial will be about 1.17”.