Quis custodiet ipsos custodes? You have to be able to trust your original demon not to lie about that, and you can't verify him. You only can if you have Sense Holiness and Unholiness, a quirky divine power that makes you a shoo-in for Quaesitor.
The other option is to summon the possibly-angel into a Circular Ward against Demons. If it's an angel, it will be able to step out or leave whenever it wants to. If it's a demon, it can't. (Unless it can overpower the ward, of course.)
And, as mentioned previously, the other option is to use the low-power, high-penetration DEO a demon detection device. If it's an angel, it will understand the reasoning. If it's a demon, it will start dissolving.
Hm. You're right. I just re-read the RoP:I page describing the interaction between demons and magic. I had forgotten that demons also can't be compelled to reveal truthful information through, say, ReMe. Core book makes no mention of anything non-Intellego/other truth-reading Ability failing to work, so I forgot that you couldn't just mentally command a demon to sense the bound being and tell you if it sensed demonic presence. I mean, the demon certainly would be compelled to sense the creature, but it can still lie through the Rego spell.
Shoot. I guess an alternative is to mentally dominate an appropriate diabolist, but that likely won't be a lot of help either because low-level Incantation/Psychomachia can't tell the Divine and Infernal apart, high-level versions don't have a chance of Penetrating any but the wussiest demons, and Ablation is just difficult because any Goetic Summoner strong enough to make this particularly effective will present serious problems with the usual plan of "acquire and fix AC, dominate" that works so well on wimpier people.
Anyway, I failed here. Dang.
It does seem true that developing summoning spells and spells to control an NPC with a high skill level in sense holiness and unholiness would be a great deal easier (even if having less potential power).