In my opinion you have two options:
Just try to convert it to an int, but catch the exception:
try:
value = int(value)
except ValueError:
pass # it was a string, not an int.
This is the Ask Forgiveness approach.
Explicitly test if there are only digits in the string:
value.isdigit()
str.isdigit() returns True only if all characters in the string are digits (0-9).
The unicode / Python 3 str type equivalent is unicode.isdecimal() / str.isdecimal(); only Unicode decimals can be converted to integers, as not all digits have an actual integer value (U+00B2 SUPERSCRIPT 2 is a digit, but not a decimal, for example).
This is often called the Ask Permission approach, or Look Before You Leap.
The latter will not detect all valid int() values, as whitespace and + and - are also allowed in int() values. The first form will happily accept ' +10 ' as a number, the latter won't.
If your expect that the user normally will input an integer, use the first form. It is easier (and faster) to ask for forgiveness rather than for permission in that case.