First and foremost, you could start by reading reviews of several assisted living facilities. By doing that you can narrow down your options, and thus come up with the best reviewed assisted living Denver locations. Reviews can mention many important details, including those about the staff members at an assisted living, but also about food, hygiene, good or bad experiences, and so on. In this respect, if you find out anything that refers to neglect or abuse, you surely must move on to the next facility.
However, reviews are not enough. You should definitely make a tour of the place, before deciding whether or not you should bring your senior parent to stay there for a longer period of time.
Talking to the staff is another important aspect. It is not too much to expect staff members to treat your parents with the same care and attention that you do. Although this may seem like a high standard, you certainly can look for signs that staff members have a raised level of compassion and empathy towards elderly people.
In case any of the reviews you read mentioned a conflict, you can ask staff members about that, and thus discover their way of solving such issues.