I'm glad you asked, it prompted me to add ISARRAY to my own code libary like this:

FUNCTION ISARRAY( tcVar)
RETURN TYPE("tcVar[1]") <> "U"

For ISARRAY() to work you need to pass the variable by reference. For example:

myVar = 3   && a variable
dimension myArray[1]    && an array
myArray[1] = 3
?ISARRAY( @myVar)   && returns .F.
?ISARRAY( @myArray) && returns .T.