Here's advice I've gotten from people who have already gone through this process:
Get a clear sense of your financial package. (Guaranteed funding for 5 years? TA/RA strings attached? Is 6th year funding possible? Health care provided?). Make sure to compare financial package with cost of living.
Get a sense of the faculty members, research groups, or labs that you could work with. Ask yourself: Do they form a supportive community for you to do your best work?
Ask about graduate placement if they don't already have that information published. If graduate placement is published, I would still investigate more (look at CVs, reach out to recent grads, etc.). Some placement documents I've seen won't differentiate between post-doc, TT, or adjunct jobs which can be very misleading.
What is the program/cohort like? What proportion of admitted students finish their PhD? How many years is the typical student taking to complete their PhD?
Will you get a lot of individual attention as a grad student, or will you be working more on your own? Pick a program that aligns with how you like to work. I've heard UC schools are usually more of the "fend for yourself" variety, but I could be wrong.
If you are planning on a career outside of academia: will faculty be willing to help you with your career goals? Is there a stigma associated with students who pursue non-academic careers? (I probably wouldn't ask this outright, unless you're sure you want to work outside of academia in which case it might be good to bring up early.)
Reach out to PhD students currently in the program. Are they happy? Idk if this is true, but someone told me that if they are too happy expectations might be too low. Obviously if they're miserable you have to decide if you want to submit yourself to that for the next 5+ years
Hope this helps!