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It is best to always keep track of the assumptions that you and your team are making about your product through the life of the product. During the product sprint this should be done throughout all phases. It is best to analyze and figure out how to test the assumptions during Converge.
Have a dedicated space to collect Assumptions on the whiteboard.
Throughout the sprint have everyone write down assumptions that they are making or that they hear other people making and put them up on the whiteboard.
At the end of Converge, create a table with 3 columns: Assumption, Test, Validated if.
List out all of the assumptions that team is making.
For each assumption decide how you plan on testing to see if the assumptions are valid or not and how you know if that assumption is valid.
Repeat till all assumptions have a test for them.