assuming you have a string that's just a comma-separated list of numbers like:
var nums := '100, 101, 110, 1000, 1010, 1100, 200, 210';
just do something like this:
var slist := TStringlist.Create;
slist.CommaText := nums;
if slist.IndexOf( '101' ) > 0 then
// it's there
else
// it's not