this is going to sound a little dorky, but: read it out loud to yourself! try to imagine a conversation back and forth, how you might reply, what key words lead to what kind of responses (or questions!), etc.

usually, if i'm struggling with dialogue (or if i want dialogue to get across a specific plot point that won't work anywhere else in the fic), i'll write something that might not even sound realistic, and then i'll come back to it later. then i read it out loud and try to see if i can come up with alternate phrasings!