It's pretty easy to check:
string s = "25%";
if (s.Contains("%"))
{
Alternatively:
string s = "25%";
...
s = s.Trim();
if (s.EndsWith("%"))
{
But...That codes a bit...poor.
Don't use Convert.To - use Int.TryParse or Double.TryParse instead.
They give you a bool result which tells you "this wasn't a valid integer", or "this wasn't a valid double" and allows you to tell the user there is a problem instead of continuing with possibly the wrong values and the user doesn't know it.