15 years post op - how would I know if I still have internal stents in my nose? Are stents used for every rhinoplasty?

Occasionally, stents are used to stabilize the nose/septum after septoplasty/rhinoplasty. They are generally a rubberized plastic material. It's highly unlikely that you had them placed along with gauze in the nose as the gauze is generally placed for the same purpose. Stents, if placed, are sutured in place to avoid displacement, a patient pulling them out, or swallowing them! I have seen cases of stents, or similar things, left in the nose for a long period of time. After years, they generally become more brittle and can crack or break. I have seen cases where patients get a lot of crusting (mucus build-up) on such things left in the nose.
Given all of that, I feel it's extremely unlikely that stents are still present in your nose after 15 years. A quick check by an otolaryngologist or nasal plastic surgeon will ensure that and give you absolute peace of mind.

Stents are often utilized in nasal surgery to provide support and aid in correct healing. Internal nasal splints/stents are used by many surgeons, however they are not used by everyone. Furthermore, there use is situationally dependent. The best way to know if there is any material in your nasal cavity is to have the area examined by your healthcare provider.

Most likely you would know because you can see them! If you have any doubts, go see an ENT who can scope you in the office. I heard a crazy story from Europe that one patient had internal splints in for 12 years so it can be well tolerated! Best Wishes!!