First you can calculate you body mass index, or BMI. The BMI is calculated from the formula kg/m2 - that is your weight in kilograms divided by your height squared. Commonly accepted BMI ranges are underweight: under 18.5, normal weight: 18.5 to 25, overweight: 25 to 30, obese: over 30. But of course age and weight should also be taken into consideration
If you can measure your height and weight, there is an online tool in the Resources list that does this easily for you - all you need to do is to fill in your age, sex, height and weight and you will be given an assessment of whether your weight is significantly too high or too low for your height.
Next, try to determine if there is objective quads muscle wasting on the affected side. Take a pen, mark a point two inches (or five centimetres) above your kneecap, and measure the girth at that point. Compare this with the good side.
Doing simple exercises for your quads (eg quad sets) can help to build them up before surgery, which means that your rehab will have a better starting point.
The final challenge is to compare the range-of-motion of the two sides. Lie on your back on a firm flat surface, and bend your knee, pulling your heel up towards your buttock. Ask a helper to measure the closest distance you can achieve for this heel-buttock measurement. Compare the two sides. This will give you an idea of any lack of flexion.
Rolling onto your stomach on a bed, and allowing both legs to hang over the edge will give you some idea of the extension that you can achieve, and you will be able to compare the two sides.
Resources
Checklist of fitness things to think about before surgery - to download and print
Knee exercises for knee pain - Isometric Quads