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”.
Locate Animals or Plants
The spell locate animals or plants can tell us what need to know:
Describe or name a specific kind of beast or plant. Concentrating on the voice of nature in your surroundings, you learn the direction and distance to the closest creature or plant of that kind within 5 miles, if any are present.
With this one, if you suspect a nearby cat may be a familiar, cast this spell and concentrate on cats. If the suspicious cat pings with the spell, you know it isn’t 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)