Use PropertyInfo.PropertyType to get the type of the property.
public bool ValidateData(object data)
{
foreach (PropertyInfo propertyInfo in data.GetType().GetProperties())
{
if (propertyInfo.PropertyType == typeof(string))
{
string value = propertyInfo.GetValue(data, null);
if value is not OK
{
return false;
}
}
}
return true;
}