How do I check the carb on my lawn mower?

If your carberator is shooting smoke then you need to have ti repaired immediately, but chances are no this silver box is the muffler not the carberator. Your carberator would be located either bolted to your gas resevoir (some older briggs&stratton engines) or inline with yoru fuel perhaps bolted to the lawnmower with a fuel line running from the fuel tank. In all cases yoru carb is located behind yoru air cleaner housing and usualy this is indicated by either a very long bolt with a large standard screwdriver head or a wing nut, once the wingnut is removed or the long screw usually the air filter housing can be simply lifted away and off of the lawnmower exposing the float valve to the carberator, if you are new to this kind of stuff as I assume you are from the nature of the question I would not attempt dissasembly of the carb itself as these are very delicate and less then easy to diagnose by a very long shot, more often then not you can spray some carb cleaner into the carb a few time and run the mower and this often solves the problem y releasing stuck valves or even a stuck float this carb cleaner is available at most major auto parts stores for less then 3 dollars (for a big can). Please do not spray this into the silver box that makes smoke because carb cleaner is highly flammable and spraying it into the exhaust is a sure fire way to set yoru mower ablaze.

The silver box is the muffler (don't touch it because it's HOT!)

The carb is most likely on the other side. For someone that has little experience with carbs it is hard to give full lessons on the inner workings of carburation over the computer. My suggestion is if it runs okay don't fix it! If it doesn't run, have a qualified technician repair it for you. This will save you time, money, and skinned fingers!

If you don`t know where it is, What do you plan on doing when you find it? No the silver box that shoots out smoke is the muffler. There is an air filter you could possbly change yourself, but then you would find the carb, and I won`t tell you .