A Sonnet...by any other name?

We looked at different forms of Sonnets at poetry class and then looked at two - a Keats and a modern sonnet  to compare and decide whether each had kept to the formula.  We decided that Keats just about did (he had one dubious rhyme) and that the modern one also did even though it was set out differently, however it begged the question - when is a Sonnet not a Sonnet?  It seems poets go to great lengths to take the formula apart so that it can be difficult to tell and perhaps the only thing left is that it has 14 lines (for a Shakespearean Sonnet).  I have often come across judges who say 'this is a Sonnet' and I'm thinking "eh? How come?"  I can't be bothered to analyse the form that far, I just like a poem or I don't.  Maybe that's where I go wrong.  I guess it's like art once you have learned how to paint you can then do something a child could do!  Is that a bit harsh?  I think I need to learn more so I can then say of mine, "well of course it's a Sonnet, it has a turn and it sort of rhymes"!!  Thoughts anyone?

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