Ask HN: How can you tell fear from useful gut feeling?

This is a broad question, but I'll give you an example based on my situation: I'm working with a client as IT/marketing consultant. I've had a strange feeling about it since day one, but it's my main source of revenue and I ignored it. In this project I'm also stretching my capabilities - I'm doing some things I've never done before, I want to grow as a professional so I thought the gut feeling was just fear of getting out of my comfort zone. Now I'm torn: the strange feeling is growing by the day, and I'm not sure whether it's because I don't know whether I can deliver on some objectives or because I don't like my relationship with the client. Do you have any key questions you ask yourself in this case? Any useful advice? Thanks.