getUser API can be used to find out if a user is online and details like profile link, avatar and status message.

API:

jqcc.cometchat.getUser('USERID','CALLBACKFN')

Sample:

1. To check if user status

window.onload = function() {
    jqcc.cometchat.getUser('1', 'checkstatus');
}
 
function checkstatus(data) {
    console.log(data); // To see output using Firebug
    if (data.s == 'available') {
        alert('User is online');
    }
}

2. To get all details of a user

function details(data) {
    alert(data);
}
jqcc.cometchat.getUser('1', 'details');