Body Mass Index is one of the more frequently used statistical means of determining how 'ideal' a person's weight is. However, being based purely on height and weight, it cannot differentiate between adipose tissue and muscle tissue...

In metric, mass (kg) / (height (m) ^ 2)
In imperial, mass (lb) x 703 / (height (in) ^2)

The wiki article also includes a chart to give you an at-a-glance guide to which mass / height ratios fall into which category, as defined by various people within the medical establishment. An interesting (and easier to calculate) alternative is the WHR (Waist to Hip Ratio).

Waist circumference / Hip circumference

Apparently figures of around 0.7 for women and 0.9 for men are considered healthy. Wiki also mentions SAD (Sagittal Abdominal Diameter), which translates as the distance from one's back to one's upper abdomen, midway between the top of the pelvis and the bottom of the ribs, measured while standing (<10" is apparently healthy, >12" is apparently unhealthy). Don't ask me how you'd go about measuring it - the helpful diagram on the Wiki article shows someone lying on the floor...

Obviously everyone has their own idea of their preferred mass, and these statistical measurements are only a guide (they don't compute exactly how much fat you've got). It's possible for people to have apparently similar diets and quantities of exercise but also have completely different body shapes. It's all a matter of personal preference as to whether you decide you need to gain or lose weight, and if so, if you're going to use these statistical methods as a means of progress measurement.

And on the subject of eating, most manufacturers of processed foods have now decided we need help in determining which foods are healthy and which aren't, so (in the UK at least) have started displaying statistics on the 'naughty stuff' (calories, fat, saturated fat, carbohydrates, sugar, salt) within on the front of the packs. Some go by GDI (what percentage of your recommended maximum daily intake a portion provides), while others have opted for 'traffic light' guidance (which is based more on percentage of naughty substance within the product - so a small 'red' cupcake may contain fewer calories than a 'green' salad). As if that wasn't bad enough, the UK's FSA (Food Standards Agency) has decided that's not enough, and is currently trying to persuade restaurants to dish out traffic light guides to their meals...