var key = "tester";
var val = "testing";
if(myDictionary.ContainsKey(key) && myDictionary[key].Contains(val)) 
{
    // "tester" key exists and contains "testing" value
}

你也可以使用TryGetValue：

var key = "tester";
var val = "testing";
var dicVal = string.Empty;
if(myDictionary.TryGetValue(key,out dicVal) && dicVal.contains(val)) 
{
    // "tester" key exists and contains "testing" value
}