There are many signs that point to lifters going bad. A knocking sound is most common, of course. If you suspect the lifters are bad, which is a common failure in relatively high mileage engines, you can have them checked by a professional or pull them yourself and see.
Removing the lifters can be an easyor a very involved job, depending on your vehicle. In general, smaller engines will allow you to access the lifters by removing only the valve cover while larger engines may require the entire head be pulled in order to get to them.