Find id of element on already made google form?

2021 update:

To find the ID manually:

1. Inspect question element
2. Find [data-params] attribute
3. The second number will be the right one

Example:

data-params="%.@.[860146445,"Question",null,0,[[305915825,[],true,[],[],null,null,null,null,null,[null,[]]]],null,null,null,[]],"i1","i2","i3",false]"

305915825 - this is the field ID that you can use like ?entry.305915825=<your-predefined-content>

Something like this will show you the ids in the logger...

var form = FormApp.getActiveForm();
var items = form.getItems();
for (var i in items) { 
  Logger.log(items[i].getTitle() + ': ' + items[i].getId());
}

To see the IDs manually in Chrome >> View Form >> right-click on the question Label >> Inspect element >> look for the for="entry_XXXXXXX" attribute. That 'xxxxxxx' will be the id.