#JavaScript (*ES6*) 21 <s>27</s>

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The rule: `(y%4==0) && (y%100!=0 || y%400==0)`

Golfed: `!(y%100<1&&y%400||y%4)` (mainly using [De Morgans's law][1])

**Edit** -6 chars, thanks David Hammen `!(y%(y%25?4:16))`

A function implementing the rule:

    l=y=>!(y%(y%25?4:16))

A test (run in Firefox) just to be sure:

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    l=y=>!(y%(y%25?4:16))

    for(o=[],i=0;i<700;i++)
      y=i+1800,
      x=l(y),
      o[i/100|0]=(o[i/100|0]||'')+y+(x?' <b>Y</b>':' <i>N</i>')+'\n'
        
    R.innerHTML='<td>'+o.join('</td><td>')+'</td>'
    console.log(o[1])

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    td { white-space: pre; font-family: monospace; padding: 8px}

    b { color: red; }
    i { color: blue; }

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    <table>
      <tr id=R></tr>
    </table>

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  [1]: http://en.wikipedia.org/wiki/De_Morgan%27s_laws