If you need to check the imei number on your galaxy s6 you first need to go into the settings. Next scroll all the way down to the bottom where it says about device, and then click on status. From in here you will be able to find the imei number on your galaxy s6. As far as checking the imei number for your galaxy s6 or a galaxy s6 your looking to purchase stay away from swappa. This is literally the most inaccurate site for checking an imei number. One of the best free sites is freeesnchecker.com.
This site has most of the major providers as well as some secondary carriers. Even after you check your imei number its always best to still give your provider a call to double check its a clean imei number. Most of the major carriers will do this for you, and they have gotten pretty fast at doing it. If there is any major points to take away from this post it would be to stay away from swappa, use freeesnchecker.com, and if your not sure then just call your provider to confirm the imei number for your samsung galaxy s6 is clean.