How will the question of deciding whether or not trying the impeachment of a former president is constitutional?

precedent exists ... search the internet and you'll find it.  Anyway, without Justice Roberts presiding, you can be sure that the chair will rule it is constitutional

Although Trump was never an actual president he was in office when he was impeached.

Impeachment is more of a "political" event than a "criminal" event.  The Supreme Court has already said that they will not get involved in any impeachment proceedings for that reason.

The Senate needs a 2/3rd majority vote to find a President guilty in an impeachment trial.  Even if the Senate does have a trial it's unlikely that 2/3rds of the Senate will vote guilty.

How can it be unconstitutional, when the Constitution EXPLICITLY gives the House the right to impeach the President, mandates that the Senate Tries him if they do, and to do it pretty much any way they want?