I'm not necessarily requiring the scrying spell (because it is limited to the plane you're on), but is it possible to get a similar effect of seeing/hearing through a sensor/creature/something else on another plane?
The goal is to send a creature to the Nine Hells and verify what's on the other side without risking the life of a humanoid.
I am looking for solutions that are standard options in the rulebooks and not requesting homebrew or houserule solutions.
Casing beast sense on a familiar or other willing beast and sending it to the target plane would allow the caster to perceive the destination in another plane.
Negotiating with a creature to act as a spy via planar ally is a viable option. The spell description explicitly lists "spy on our enemies" as something that one might request.
Having a familiar go to the plane and then cast sending on it is a method to get some information about the destination without risking a humanoid.
You can send the message across any distance and even to other planes of existence, but if the target is on a different plane than you, there is a percent chance that the message doesn’t arrive. (PHB, Sending spell; description)
If the information your seeking is sufficiently specific that it can be obtained with a yes or no answer, using commune to ask your divine power may work.
Another ask an extra planar entity could potentially obtain the information by means of the contact other plane spell.
This only meets part of your requirements, since it does put a humanoid in the line of fire (though it doesn't have to be a friendly humanoid).
It's maybe not RAI, but a Warlock with the Gaze of Two Minds Eldritch Invocation can see through the eyes of a creature on another plane for a single round.
You can use your action to touch a willing humanoid and perceive through its senses until the end of your next turn.
The restriction on seeing across planes is only on the renewal clause:
As long as the creature is on the same plane of existence as you, you can use your action on subsequent turns to maintain this connection...
You have to do the following:
You can get a little more out of this by using a Fighter as the humanoid you are looking through, and having them Action Surge to get an action on the far side of the portal.
