What would be the best way of evaluating the performance of an inner query?

say you have something like

SELECT Key, sum() , sum() max()

into #table

from

(select key, case(), case() case()

union

select key()case, case(), case()

) as InnerQuery

group by key

say you have concerns of the innerquery could be done better, would doing a select into temp table from the inner query be a reflection of the performance in this instance? or is the no true way of evaluating it?