isfunction(FUN)

returns true for valid matlab functions (v3.2, apr 2018)

TF = ISFUNCTION(FUN) returns 1 if FUN is a valid matlab function, and 0 otherwise. Matlab functions can be strings or function handles.

[TF, ID] = ISFUNCTION(FUN) also returns an identier ID. ID can take the following values:
1 : FUN is a function string
2 : FUN is a function handle
0 : FUN is not a function, but no further specification
-1 : FUN is a script
-2 : FUN is not a valid function m-file (e.g., a matfile)
-3 : FUN does not exist (as a function)
-4 : FUN is not a function but something else (a variable)

FUN can also be a cell array, TF and ID will then be arrays.

Examples:
tf = isfunction('lookfor')
% tf = 1
[tf, id] = isfunction({@isfunction, 'sin','qrqtwrxxy',1:4, @clown.jpg})
% -> tf = [ 1 1 0 0 0 ]
% id = [ 2 1 -2 -4 -3 ]

See also function, script, exist, isa, which, nargin, function_handle