The Excel Islogical function tests if a supplied value (or expression) returns a logical value (i.e. evaluates to True or False). If so, the function returns TRUE; Otherwise, it returns FALSE.
The syntax of the function is:
Where the supplied value argument is the value or expression to be tested.
The following spreadsheet shows examples of the Excel Islogical function:
Formulas:
 Results:

Note that the Islogical function in cell A3 would still return TRUE, even if cells B1 and B2 were not equal, as the Islogical function is simply testing if the expression 'B1=B2' evaluates to any logical value.
Further details and examples of the Excel 'Is' functions are provided on the Microsoft Office website.