public int GetItemFromList() {
	List<Item> list = new List<Item>(
      new Item(1),
      new Item(2),
      new Item(3)
    );

	Item testItem = new Item(1);

	// Inside FindIndex() you can specify a lambda expression where you
	// query if an item exists like a boolean test.
	int index = list.FindIndex(item => testItem.Id == item.Id);

	// in this case out testItem.Id (1) is equal to an item in the list
	if (index > -1)
	{
		// We get here with the index 0!
      	return index;
	}
}

public class Item
{
	public int Id { get; set; }
	public Item() { }
	public Item(int id)
	{
		Id = id;
	}
}

List<int> nums1 = new List<int> { 2, 4, 6, 8, 10 };
List<int> nums2 = new List<int> { 1, 3, 6, 9, 12};

if (nums1.Any(x => nums2.Any(y => y == x)))
{
    Console.WriteLine("There are equal elements");
}
else
{
    Console.WriteLine("No Match Found!");
}

var items = (from x in parameters
                join y in myStrings on x.Source equals y
                select x)
            .ToList();

You could use a nested Any() for this check which is available on any Enumerable:

bool hasMatch = myStrings.Any(x => parameters.Any(y => y.source == x));
Faster performing on larger collections would be to project parameters to source and then use Intersect which internally uses a HashSet<T> so instead of O(n^2) for the first approach (the equivalent of two nested loops) you can do the check in O(n) :

bool hasMatch = parameters.Select(x => x.source)
                          .Intersect(myStrings)
                          .Any();