(I’m going into more detail than the OP needs, because I figure clarifying all of the relevant rules and guidelines will be useful to new players who might come across this thread.)
Relevant rules include:
-Visibility rules (including distance) in the DMG on pages 117, 119, and 243.
-Tables on the DM Screen Reincarnated for visual encounter distance based on terrain, and hearing range based on whether creatures are making normal noise, trying to be quiet, or being very loud. Based on the language on page 243 of the DMG, these were probably meant to be in the DMG but cut for space.
The encounter distance tables appear to assume uneven terrain or potential obstructions, because none of them can generate results greater than a few hundred feet. The DMG visibility distances tables, OTOH are explicitly maximum distances to see in open, unobstructed terrain.
Within visual range (limited to the encounter distance tables on the screen when not completely open and unobscured) you automatically notice someone if you are alert (not distracted or occupied in another task—and based on the general rules presentation and adventures with examples of lazy guards not even getting a chance to notice you, you have to be choosing to be alert, it shouldn’t be assumed everyone always has the benefit of their Passive Perception* up outside of combat) and they are not attempting Stealth.
That frames the relevant context. As to the actual question about discerning detail of these creatures that have been detected, the only rules are the general guidance on setting DCs for tasks where you judge success to be uncertain.
In this specific kind of situation, I’d consider the size of the creatures, how much their appearance differs from other races, and any other potentially identifying factors. Then, I’d either rule auto success or failure, or go with a gut feeling on a DC. I very much like the suggestion of using Intelligence rather than Wisdom for the Perception check in this case (alternate abilities with skills is a suggested variant/option right in the PHB / Basic Rules). I recognize that whether or not someone is comfortable with 5e’s encouraged manner of setting DCs more or less on the fly is a personality thing as well as an experience thing. The most experienced and intelligent DMs don’t always find that method fits them. Since it does fit my personality style it is a big plus for the game to me. I’d encourage deciding whether you like doing it that way or not. If so, go with it. If not, go with a more detailed table like one of the ones suggested on this thread. I’d just encourage you to keep it within the bounds—and meaning—of 5e DCs. For instance, DC 30 is “nearly impossible”. If single number increments are too granular, you might stick with increments of 5, or even just five options: Automatic Success, DC 10 (Easy challenge), DC 15 (Moderate challenge), DC 20 (Hard challenge), Automatic Failure.
* Jeremy Crawford said your Passive Perception is always active. However, if he was not referring specifically to combat (or referencing situations where you are allowed to make an active check and clarifying that your PP is the minimum, which if I recall was the context), he was mistaken as far as the rules he published. It happens. The actual rules are better and make it so that the typical person doesn’t automatically notice every typical pit trap they come across and every typical concealed door in a hallway they walk down, while allowing PP to be automatically up causes both scenarios to be true.
