How to determine sign across current source?
The sign you assign to the voltage across the current source has no bearing on the analysis. If you think that getting a negative value of voltage across the source is somehow "incorrect" then calculate first and flip it later.

You would tend to assign the voltage polarity in a way that the voltage you get in your analysis is positive. But in some circuits, particularly those with dependent current sources, this might be hard to determine before you do the analysis. In other circuits, a particular voltage polarity might be desirable for some other reasons.

Experienced designers might see deeper into the circuit, so their assignments might not be so obvious to someone that lacks the experience.

In that particular circuit it is rather clear to me, without doing the actual analysis, that the polarity is "correct." As in it will lead to a positive value for V.