What are some ways to measure traction?

First, product/market fit can only be determined after there are customers using the product and paying for it.  It would be entirely too hypothetical to make claims about product/market fit based on customer research or unpaid product testing.

Assuming you did get in the market and are running a paid pilot program or are already in market, you will have an install base of customers who have paid for your product and are using it with various levels of customer satisfaction and loyalty.  In that event, there are two tests you can run against your install base that will give you an indication of product/market fit:

• Asking the product/market fit question: "On a scale of 1-5 where 1 is not at all disappointed and 5 is very disappointed, how would you feel if we take this product or service away from you and you no longer have access to it?"  Product market fit is achieved when you get 40% of respondants answer 5.
• Asking the Net Promoter Score (NPS) question: “How likely is it that you would recommend [your product] to a friend [or family, or colleague etc]?” and apply a standard formula to calculate the NPS.   Read more about NPS here.