Making code suitable for unit tests

In this code the user presses a button cmdPythagoras which calls the FindAllTriples method.  How would I unit test this code?  The FindAllTriples method doesn't return a single value.

`````` private void cmdPythagoras_Click(object sender, EventArgs e)
{

FindAllTriples();
}
public void FindAllTriples()
{
try
{
List<Pythagoras> pt = new List<Pythagoras>();
int Small = 0;
int Med = 0;
int Big = 0;
CheckPrimes cp = new CheckPrimes();

for (Small = 2; Small <= 100; Small++)

{
for (Med = Small + 1; Med <= 1000; Med++)
{

{
for (Big = Small + 1; Big <= 1000; Big++)
if ((Small * Small) + (Med * Med) == (Big * Big))
{
//Example using delegate
myDelegate dg = new myDelegate(cp.CheckIfNumberIsPrime);
Pythagoras p = new Pythagoras();
p.Smallest = Small;
p.Medium = Med;
p.Biggest = Big;
if (dg(Small) == true || dg(Med) == true || dg(Big) == true)
{
p.Comment = "At least one prime";
}
else if (cp.GCDRecursive(Small,
Med) == 1 && cp.GCDRecursive(Small, Big) == 1 && cp.GCDRecursive(Med, Big) == 1)
{
p.Comment = "Numbers have no highest common factor";
}
else
{
p.Comment = "";
}
}
}
}
}
using (StreamWriter sw = new StreamWriter(@"D:\Documents\MathsPrograms\Pythagoras.csv", false))
{
foreach (Pythagoras p in pt)
{
sw.WriteLine(p.Smallest + "," + p.Medium + "," + p.Biggest + "," + p.Comment);

}
Environment.Exit(0);
}
}

catch (Exception ex)
{
Console.WriteLine(ex.Message);
}
}
public Boolean CheckIfNumberIsPrime(int CheckThisNumber)
{
int Counter = 2;

Counter = 2;
//for (Counter = 2; Counter*Counter <= CheckThisNumber; Counter++)
do
{
if (CheckThisNumber % Counter == 0)
{
return false;
}
else
{

}
Counter += 1;
} while (Counter * Counter < CheckThisNumber);
if (Counter * Counter > CheckThisNumber)
{
return true;
}
else
{
return false;
}

}
public int GCDRecursive(int a, int b)
{
//article on http://www.vcskicks.com/euclidean-gcd.php
if (a == 0)
return b;
if (b == 0)
return a;

if (a > b)
return GCDRecursive(a % b, b);
else
return GCDRecursive(a, b % a);
}
}``````

I wrote this unit test for the GCDRecursive method.
``````using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using MathsProg;
namespace UnitTestProject1
{
[TestClass]
public class UnitTest1
{
[TestMethod]
public void TestHightestCommonDivisor()
{
int FirstInteger = 20;
int secondInteger = 24;
int expected = 4;
CheckPrimes cp = new CheckPrimes();
int actual=cp.GCDRecursive(FirstInteger, secondInteger);
Assert.AreEqual(expected, actual);

}
}
}``````