Body mass Index (BMI) has long been the standard way to gauge if someone is overweight. It's a simple formula. You just divide your weight in kg by your height in meters squared. 
So if you are 1.7m tall and weigh 70kg
Square your height: 1.70 × 1.70 = 2.89
Divide weight by squared height: 70 ÷ 2.89 = 24.2 

You can then use the number to see where you fall on the scale below:
Underweight: Below 18.5
Healthy weight: 18.5 to 24.9
Overweight: 25.0 to 29.9
Obese: 30.0 and above

Lately it's been criticised as it doesn't really give a useful output for many people, if you are fit for example, as muscle weighs more than fat. Many fit people will find they erronously fall in the overweight category.

A more basic question may be why should you care? The answer is that excess weight can lead to a whole host of other health problems: Structural issues like joint wear from the extra weight.  The soon to be topic-of-the-moment, increased inflammation, caused by excess body fat. Excess body fat, particularly when concentrated around the abdomen, is also associated with increased mortality risk. This is largely through its effects on blood pressure, cholesterol, blood sugar regulation, and blood vessel health, which raise the risk of heart attack and stroke.

But if BMI isn't great at predicting things like cardio risk then what is better? Turns out that Waist to Hip ratio (WHR), or even the oddly definitive Waist Circumference (WC) is a far better indicator. This is because the real risk if you are overweight is carrying fat around your middle. (There's also the white / brown fat issue that we'll leave for later) . So if you want a quick check of how your diet and weight may impact your mortality then checking your figures against these thresholds might be useful:
Waist-to-hip ratio (WHR) — "high" defined as:

• Females: > 0.85
• Males: > 0.90

Waist circumference (WC) — "high" defined as:

• Females: > 88 cm (~34.6 inches)
• Males: > 102 cm (~40.2 inches)
  
  

Though there is a newer, and maybe easier to remember / measure guideline, and that's waist to height ratio (WHtR). Simply divide your waist measurement (CM) by your height (CM). Anything over 0.5 and you could probably do with looking at some lifestyle changes if you want the best outcomes.

It's a newer measure and seems to be a U shaped distribution (very small waist to height isn't great either) and it seems to suggest over 0.58 is the real inflection point.

We like this measure more as it at least takes some account of frame size unlike the single measurement waist size, and is probably easier to estimate for most people.

'Health' can be very complex so its good to have some easy. basic concepts to build off. The waist to height ratio is one of the ones we like. 

Studies below if you want to go deeper.

https://pmc.ncbi.nlm.nih.gov/articles/PMC13470613

https://pmc.ncbi.nlm.nih.gov/articles/PMC12375492/