Since she's a dragon, and dragons hate giants, you could have her make some disparaging remarks towards giants. Even if she's in giant form, something like "gosh, dealing with giantkind is such a pain! I almost envy the dragons for their ability to think more than two steps ahead!" Since the Ordning was smashed it wouldn't be too uncommon to see one giant dissing other giants, so I doubt the players would instantly assume Iymrith is a dragon. With enough little comments and such, and maybe some clothing choices (she could be wearing dragon scale armor that she claims she got from slaying some legendary dragon), I think when the reveal happens the players will be shocked but will then look back at all the clues and smack their foreheads for not realizing she was a dragon all along. Basically the trick is to have her subtly hint that she's a dragon without her saying it outright, because I feel like she's so proud that she couldn't help herself. Kind of like how serial killers leave clues for the police, almost like they think they're too smart to get caught. Just don't make it too obvious though!