How to validate a dropdown menu using JavaScript?

I have three drop down menus in a single page. I want to validate all the drop down items in a single click. Any suggestions would help.

Here is my code for a drop down validation:

HTML

<select id="ddlView">
    <option value="0">Select</option>
    <option value="1">test1</option>
    <option value="2">test2</option>
    <option value="3">test3</option>
</select>  

<input type="button" onclick="Validate()" value="select" />

JS

function Validate()
{
    var e = document.getElementById("ddlView");
    var strUser = e.options[e.selectedIndex].value;

    var strUser1 = e.options[e.selectedIndex].text;
    if(strUser==0)
    {
        alert("Please select a user");
    }
}