visit Html5 Storage Doc to get more details. Use the following syntax to set and get the values in localstorage/sessionstorage
for storing values for a session

sessionStorage.getItem('key')

sessionStorage.setItem('key','value')

or store values permanently using

localStorage.getItem('key')

localStorage.setItem('key','value')

and u said u want to know the value but you want key then you can use the function

  localStorage.key(i);

or also you can loop through the available keys and get the desired key by cross checking the value

for(var i=0, len=localStorage.length; i<len; i++) {

    var key = localStorage.key(i);

    var value = localStorage[key];

    if(value.equals(desired_value))

    console.log(key +  =>  + value);

}