As someone who did watch all of Xena and didn't watch Hercules, there's only one single crossover point that matters in that direction:

At the end of one episode, Callisto is in Tartarus. Next we see her, she's immortal, trapped in a cave, and going slightly insane. The events that led to this happened in an episode of Hercules.

While there's likely references and other small things, that's the only one that made me go "Wait, what?" and have to head to the hercxena wiki.

As someone who did watch all of Xena and didn't watch Hercules, there's only one single crossover point that matters in that direction. Between Xena 2x07 (Intimate Stranger) and Xena 2x14 (A Necessary Evil) is Hercules 3x12 (Surprise):

At the end of Intimate Stranger, Callisto is in Tartarus. Next we see her in Xena, she's immortal, trapped in a cave, and going insane. The events that led to this happened in the episode of Hercules.

While there's likely references and other small things, that's the only one that made me go "Wait, what?" and have to head to the hercxena wiki.