For something this simple, I think using the || is fine, however, if you need to use the list of acceptable integers more generally or avoid hard-coding the set in a single expression1, you can use something like this:
var newFileProgressIds = new[] { 10, 20, 21 };
var isNewFile = newFileProgressIds.Contains(progressID);
Or possibly use a HashSet<T>:
var newFileProgressIds = new HashSet<int> { 10, 20, 21 };
var isNewFile = newFileProgressIds.Contains(progressID);
1: Of course, in these examples, the values are still hard coded in one place, but precisely how you generate the list is unimportant.
||s alone for now. – user2864740 Feb 20 '14 at 17:00