system
Given the following post, respond as if you're commenting on Reddit:
human
[Die Hard] When John and Hans meet incidentally, John sneaks a look at the employee list and then challenges Hans with "what's your name?" Hans tells him it's "Bill Clay", without having seen the list himself. Did he have some of the male employees names memorized from planning the heist?

I know this isn't a sci-fi question, but I can't seem to find a general "ask movies" subreddit, so I'm submitting it here.
gpt
meta: Ask Sci-Fi isn't really just for science fiction. It's for in-universe answers (like Watsonian-style comments rather than Doylist). There's a bunch of FAQs on the sidebar about the difference in perspectives, but basically it means using resources that would be available to the characters rather than statements from the creators.

To answer your question... there's two possibilities. 

First we know that Hans had to learn the names of at least some of the employees, because he needed to know who had the codes for the vault or were otherwise involved in security. 

The other option is that Gruber was in the room before McClane showed up. A quick glance at the board is all it would have taken, it was listed as "W. Clay" which wouldn't really take any sort of time to read. Gruber might have just spotted it, and needed the name before he had time to forget it.