I’m wondering which is /ts/ and which is /t/ when /t/ sound comes at the end of a word. It seems like they both touch the root of the mouth and then release, and the sounds are very similar to me. I guess the /s/ may have the key to differentiate the two, but it is drowned by the /t/ to leave no /s/ trace for me. What is the difference between the two?
In fleet, the tongue does touch the roof of the mouth and release, creating the /t/ sound, and then relaxes, allowing the air exhaled during speech to pass uninhibited. On the other hand, when pronouncing /ts/, after pronouncing the /t/ the tongue remains close to the roof of the mouth with the tip of the tongue close to the front teeth as the air that is exhaled during speech is somewhat restricted, creating the /s/ sound that will help you distinguish between /t/ and /ts/.
Try pronouncing them to your self both ways to see if you can hear the difference. If this is successful, begin attempting to hear it in native speakers. If you have access to one, ask him or her to speak the different sounds, emphasizing the /t/ and /ts/ sounds.