When I went we didn't get ID'd for the first 3 days, then my other friend came over and we got asked for our ID in every coffee shop...when you walk up to the bar they will normally ask you then....

I did see a guy get ID'd and he wasn't 18, he just wasn't allowed to smoke in the shop...

If you are inconspicuous about it they might not ask you....just relax and don't go up to the bar with the people who are buying....
lol
There is nothing stopping you smoking in the hotel though....if its a smoking hotel...but I expect you dont want to be stuck in the hotel the whole time....Amsterdam is an amazing city!!