With top notch quality frames sizing is more of an art plus careful measurements and calculations. For most of us normal people the following rules of thumb are helpful.
#1) Straddle the bike in question. If it's a road bike you want at least a good inch between the top tube and the "boys". (A mountain bike you want more like 3-4 inches, but if the top tube is sloped then this becomes meaningless).
#2) For seat height on a road bike, put the pedal at the 5 o'clock position, and put you foot on it so the ball of your foot is centered on the pedal. Adjust the seat post until your leg is almost totally straight but just short of "locking" the knee. When the height is right, you should just be able to "lock" your knee by straightening out your leg with no strain or pain.
On a mountain bike you want the seat lower, and have 3-4 inches between your butt and the seat when your legs are straightened/slightly bent (for clearance when on rough terrain).
#3) Handle bars: (very subjective) With road racing bars, the top of the handlebar stem should be about an inch below the seat height. The stem extension should be so that when riding and looking down the middle of the handlebar the front axle should be blocked from view. The bar width should be your hands are in the normal place on the bars (on the brake hoods (road) or grips (MTB) when you arms are right at your sides.
These rules should get you in the ballpark about whether or not the bike you're considering is close to what you need.