I used a a really great app on my Android Tablet (Keuwl Dual channel Function Generator) to find out what frequency my T was. The App has a lot of great functions including White and pink noise Generation.
As an Electronics Engineer a word of caution about headphones, Don't use a cheap pair. Even if the manufacturer Claims they have a frequency Response of 20Hz to 20KHz this will never be a flat Response in other words the same volume at one frequency will be louder or softer at another frequency especially at the higher frequencies. Thats one reason why we use a graphic Equalizer when listening to Music. Another factor is while we may be feeding our headphones with a pure sine wave of say 500Hz what the headphones actually deliver will be our 500Hz plus harmonics at 1000 and 1500Hzand as well as sub-harmonics going down in frequency and distortion. There are of course many other factors but a good pair of headphones will better help you determine the frequency of your T and in any type of Audio Treatment you then undertake.