Type `help` in your shell to see a list of all the builtins.
Some form of builtin list is always on the man pages though.
There's no good way of telling if a builtin exists programatically (for, let's say, feature detection).
There's an almost good way with `{ PATH=; command -v isthisreallife; }` and some shells will provide a `builtin isthisreallife` that you can use without having to nuke the PATH variable.
Doesn’t the `type` command (technically a builtin) meet your requirements?
Of the mainstream packaged shells, it fails only on `posh`, but that's fine, posh is super austere, it also lacks `command`.
However, I will probably still use `command -v` for sporadic uses. That's because I don't need a subshell to capture the output:
OLDPATH=$PATH;PATH=
if command -v echo
then echo is builtin
fi
PATH=$OLDPATH
if test "$(type echo)" = "echo is a shell builtin"
then echo is a builtin
fi
case $(type echo) in
'echo is a shell builtin') do_something;;
'echo is aliased'*) do_something_else;;
'echo is /'*) do_something_else_entirely;;
esac
As a more practical suggestion, I find functionality detection is much less important since I learned to disavow all knowledge of a script when sharing it