From what I've read from others on the DIS, when you are given the final bill the receipt breaks it all down -- subtotal, tax, total. If you area on DxDP then that amount is covered as your TS. For the tip, others have stated that on the bottom of the receipt the computer already 'spits out' an 18% number and a 20% number. Now, there has been no mention of whether those numbers are based on the subtotal or the total (which would include the tip).
My DH just looks a the bottom number, which infuriates me -- especially at expensive places where the tax alone can be hefty. I don't need to tip on the tax. I tip on the subtotal, which from what I've ready in all the 'fancy manners books' is correct.
Anyway, long story short, your receipt will break it down for you so you can make your tip. The server will put your tip on your room or a credit card (that is what another poster stated) or you can leave cash.
Hope this helps. I'm sure others will post more based on actual experience but I didn't want this to go without some help.