You have your programmed track and vertical profile in your FMC. When you're on vectors what's in the box won't necessarily be correct, and that's why ATC tells you how many track miles you can expect. A good way to check if you're high or low is the 3 x alt thumbrule. If you're high you go LVL change + speed brakes, if you're low you go VS and reduce your ROD. Simple as that.