Determine function as output

How can i determine if the output is a fraction, it will return true, or else its a false. Are there any specific function?

[John D'Errico]

You want to determine if a number is a fraction? EVERY floating point number is SOME fraction. For example, 0.12345235567452?

At the very least, it is

12345235567452/100000000000000

At best, read the help for the function rat.