Put forward meaningfull proposals for what they would be doing should they be the ones negotiating Brexit. They have flip-flopped at every opportunity on Brexit. One minute they want us out of the single market, next minute it’s back in again. For example, in the 2017 manifesto they ruled out freedom of movement, then went on to sugget keeping all the benefits of the single market. The two are incompatible no matter what Labour HQ say.
Answering the question, I’d like for them to commit to a position. If they want us out of the single market then they are just as bad as the Conservative Party in this whole mess. If they us in it, and therefore accepting freedome of movement, then they should say it!