We just did this for my father-in-law.
If the person moving in is already needing a high level of care, then care quality is the absolute maximum concern.
If you are looking at a facility with multiple care levels, then be sure that each level suits, and consider how long you will be living there. Activities and an enjoyable life are most important.
It is also very important to do good financial planning.
There are independent evaluators and standardized state assessments that are a place to start. But nothing substitutes for in-person visits, interviews, and meeting the staff.
Careful financial planning is also essential.