Determine if a string is composed of numbers or characters

[HeliPilot]

I have pulled a string from an input, and I have to execute a function on it depending on if the string is numbers or characters. I tried making a function file that can determine it, however I am getting an illegal expression error C2451:

int chooseMethod (string testStr){

for (int I = 0; I<10; I++){
	if (testStr.substr(1,1) = I){
		return 2;}}

return 1;

}

Then based on the return I use the corresponding function. The error lies within

if (testStr.substr(1,1) = I)

. What am I doing wrong, or is there a common/easier way to do this?