In my experience what helped me the most is talking to other people who aren't currently in the situation nor have biases towards the person you think is gaslighting you. My ex gaslighted me intensely for months, and it was only when I talked to a friend about it that they said "Y'know, I think they're probably cheating on you" which unfortunately is honestly something I would have never conceptualized myself.
Looking back, I think for me it would be uneasiness and feeling strange about situation/interactions, but also the person's reactions to you bringing up those things. Previously we had more open communication about things, but it was different when everything I felt started to become immediately shut down (even small things). Also feeling like when you do voice your feelings, the other person turning it on you or making it feel like you're to blame.
And for me personally, it was a lot of berating me (saying that things were my fault, blaming problems we had together on me, etc), lying to me, ignoring but then also granting me like a speck of time, or communication or whatever but then going back to lying, ignoring me etc. My ex would cause a lot of arguments over things that didn't make sense and use those as excuses to ignore me. They would also project onto me about cheating and infidelity and make me feel bad for accepting compliments or giving them (this was stuff like, me seeing some person on instagram and being like 'wow they're cute' or something). I would recommend you talking to someone you really trust and laying everything out, because an outside opinion can really provide a light to the end of the tunnel.