How to detect if a certain radio button is checked with jQuery?

I'm using the jQuery Validate plug-in and need to require a field if a certain radio button is checked. How can I determine if a certain country is selected?

For example, if the Canada radio button is checked, then require this field. This isn't right, but it's something along these lines:

depends: function(element) {
    return $("input[name='country'],[value='ca'],:checked")
}

UPDATE:

I'm using @Tatu Ulmanen's code, however I'm receiving the following error after clicking outside of one of the fields that should be required when "Canada" is checked. Any ideas?

province: {
    depends: function(element) {
        return $("开发者_运维知识库input[name='country'][value='ca']").is(':checked')
    }
},

$.validator.methods[method] is undefined [Break On This Error] eval(function(p,a,c,k,e,d){e=function(...x5b|trigger|x21|x23'.split('|'),0,{}))

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$("input[name='country'][value='ca']").is(':checked');

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You can use the .is() method:

$("input[name='country'][value='ca']").is(':checked');

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$("input[name='country'],[value='ca']").is(':checked');

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here's a way:

if ($('#id').is(':checked')) {
alert('button is checked');
}

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As well as using .is(), jQuery 1.6 introduced .prop(), see http://api.jquery.com/prop/ for details.