How do I know if petrol is flowing through my carbs properly?

If the bike's running perfectly you ain't gotta check nuttin. If it's an old bike I would pull and clean or rebuild the carbs, disassemble and clean or rebuild the petcocks, check the tank for rust, add aftermarket fuel filters (always a good idea), and replace the ratty fuel lines. Just fix the bloody thing instead of pissing about.

Start where the fuel line goes into the carb, fuel flowing? Next remove the drain plug or screw on the bottom of each float bowl. Fuel flowing? Next you will have to remove and clean the jets, which may entail removing the carbs.