C/C++ :: Determine Input Type Without Converting

Originally I had to create a simple integer palindrome program that looped while the user entered 5 digit inputs (entering -1 stopped the loop). I did this using a conversion to string, reading the length to determine if the length was valid, and then reading the string forward and backwards inside of a while loop. (snippet below)

while( digitsEntered != -1)//Allow user to quit by entering -1 to end the loop
{
ostringstream convert;//conversion stream
convert << digitsEntered;//converted text from number goes in the stream
convertedString = convert.str();//store the resulting conversion to convertedString

[Code] ....

The next stage of this program was to do the same thing with strings instead of integers. However, the option to end the loop by entering -1 is still a requirement.

I think the way to do this is to first determining whether the input is a string or an integer, and if it is a string then read it and if it's an integer determine if it's -1. However, whenever I write code to do this, it converts strings to 0 so the string is not stored and cannot be read to determine if it is a palindrome. Is there a way to determine the type of input without converting it into a different type i.e. read string and then keep string or read number and keep number?