Yes floating them will show a high spine just like alum...but it' s a pain in the A$$
A spine tester is easier, but if you don' t have one, simply fletch ' em up and shoot ' em. For arrows that seem to be constantly out of group, rotate the nock to the next fletch (if you are using a drop-away it' s even better cause you can make very minor adjustments of the nock). Generally you will be able to pull most of the arrows into the group by doing this.
Let us know how it goes...