A fly reel will usually only have one handle and a permanent check, the latter makes it useless for trotting. A trotting reel must be free-running - some don't have even an optional check. It will either have two handles or none. If it's a pin, there should be a grub-screw in the centre boss of the spool, to adjust its "float"; ball-bearing reels don't have this. A decent trotter will be about 4-4.5 inches diameter, much bigger than most fly reels.
Hope this helps!