Detect Evil and Good
The spell detect evil and good will allow you to determine if a beast has a creature type consistent with a familiar:
For the duration, you know if there is an aberration, celestial, elemental, fey, fiend, or undead within 30 feet of you, as well as where the creature is located.
This works, since find familiar states:
Appearing in an unoccupied space within range, the familiar has the statistics of the chosen form, though it is a celestial, fey, or fiend (your choice) instead of a beast.
This won’t explicitly tell you “this creature is a familiar”, but if you know anything about familiars, you will know that a celestial, fiend, or fey appearing as a harmless beast will be rather suspicious; suspicious enough to say “this is probably a familiar”.
66% of the time, it works every time.
There are a few options for detecting fiends and celestials that do not detect fey. These include:
- The Paladin’s Divine Sense feature.
- Helm of the Gods (from Mythic Odysseys of Theros)