to check whether the integer is unsigned

1. If you want to know if an integer value is negative (not 'signed' - 'signed' is a property of a type, not a property of a value!), then just write

if (mynumber < 0) {
   puts ("this value is negative");
}

2. If you want to make sure that a particular type is signed (or unsigned), then use the appropriate qualifier in the declaration:

char mychar; // wrong: is it signed or unsigned?
unsigned char myunsigned; // ok, this type is unsigned
signed char mysigned; // ok, this type is signed

3. If you want to check an integral template type argument to find out whether it is a signed or unsigned type, see solution 3

4. If you want none of the above, please be more concise: please specify in the terminology of C/C++ what you mean by the terms 'integer' or 'number'; please verify that you were using the correct term when you said 'unsigned'.

Boost library has a type trait just for that:

is_unsigned[^]

By the way, I think your question was badly formulated and Solution 2 (now removed) was probably the answer to your "real" question is you want to know if a number like 18 is unsigned...