How can you truly tell your trans? I've identified as gender-fluid for just under a year and have started to question myself again. I can't tell if this is just bad dysphoria. How can you tell?

The question is how you perceive yourself, and how/if your self perception variates with time. If your identification as the opposite gender is constant, if expressing yourself and being addressed as the opposite gender makes you feel good and the same gender does not, then you're likely trans. It's all about how you see yourself. As for dysphoria, you don't necessarily have to see it as something that messes with your perceptions. You can also see this as a signal of something that your true self is trying to tell you. However, a therapist can surely help figure this out better. You can also get in contact with the trans community to share and compare feelings and experiences.

From my personal opinion, you are transgender if you, suffer from dysphoria (some don't) you identify as a gender separate from your biological gender. You may find that you wish you had a flat chest if your a Trans Man, or that you wish you could just have a bigger chest if your a trans woman, it difference from person to person, so I recommend you seek truth in yourself and how you feel and are. From there you can find your answer. As no matter how others try to outline it you are the only person who can truthfully be able to know and understand if you are transgender, there's really no box of criteria you must meet to be transgender.