Try using document.getElementById('textbox1').disabled
function CheckTextbox() {
var id;
for (var i = 1; 2 >= i; i++) {
id = 'textbox' + i;
console.log(id, 'is disabled ?', document.getElementById(id).disabled);
}
}
<input type="text" id="textbox1" disabled="disabled">
<input type="text" id="textbox2">
<input type="button" onclick="CheckTextbox();" value="Check">