In the general case, the suggestions of using passive checks (preferred) or secret rolls (eh, but okay) are good.
But I think in the specific case, where there are literally giant footprints, it's not going to take a check to guess that yep, these are giants. (I'm picturing a scene with the ranger carefully inspecting and measuring this gigantic boot-print, and then dramatically announcing "Giants!", and then everyone nods very, very slowly while exchanging significant glances.)
Favored Enemy is not usually a very powerful ability and is super situation-dependent. As a DM, it's nice when I have a situation where a player's niche feature aligns with the story. Sure, the main mechanic of this feature is advantage on specific checks, but, also: "... you have significant experience studying, tracking, hunting, and even talking to a certain type of enemy." As a DM, I would play up the fact that the ranger is sure that these are giants based on their class feature before even asking for an ability check. This isn't a spoiler — it's rewarding the character.
But that's not all — sure, they are giants, but what giants? Looking on D&D Beyond's monsters page I see dozens of listings for giants, even discounting the entries for specific NPCs or less common settings. Now that the ranger has pointed out the signs, everyone can tell that these are giants. But your ranger's special expertise gives advantage in figuring out more — frost giants? hill giants? cloud giants? ogres? trolls? dire trolls? This is what the check might give.
