Joint Space Loss
While many of these symptoms can be, on their own, attributed to non-arthritic causes, the surest way of knowing is through getting a knee X-ray. Getting an x-ray for the knee will show any internal damage done to the knee by way of the affected cartilage.
As cartilage will normally take up space around the bone, when negatively affected by arthritis, it will literally wear away, disappearing from around the joint. This is the surest method of determining whether one has arthritis, as, when compared to a healthy knee, will show a dramatic amount of empty space around the joints from where the cartilage has been worn away.