If you are in, for example, a college town, 99% of people ask for separate checks. So much so that some servers will just go ahead and separate them without even asking. Back in the real world, though, its polite to at least give a heads up when placing your order. Generally speaking, every restaurant should have the capability of splitting checks. I've heard servers say the computer won't let them do it after the order was entered in. This isn't true. Ask for a manager. Now I suppose it may be the restaurant's policy to not split checks, in which case, you can proceed however you want. I've seen plenty of people outright refuse to pay the "18% gratuity on parties of 6 or more" because they didn't agree with the "policy"..