One of the simplest ways to assess for growth and fixed mindsets is to ask people what about their work they think they could get better at and what they think they can't get better at. The former reveals their growth mindset and the latter reveals their fixed mindset.
What's important is helping them have realistic expectations about each. In some cases they need to discover they in fact can get better at things they feel are outside their control or not adaptable to learning. This discovery empowers them more in their work.