There are all sorts of work around methods to check carb balance: using a length of hose, short each cylinder and check rpm drop, etc. However, the quickest and fastest method is to use a proper air flow gauge like the one BDA recommended. I like to use a right angle adapter on mine as access can be tight in the Europa engine bay. Uni-Syns are cheap and plentiful but they strangle the air flow, instantly altering the engine idle which makes them difficult to use accurately.
Was your engine running fine before? It just needs a trim?
Yes? Then re-syncing the carbs is a fine thing to do. Get the air flow tool, uncouple the two throttle shafts, adjust the idle speed screws until both carbs are drawing the same and the idle speed is correct, and then tighten throttle shaft coupler.
Was your engine running poorly? Idle lumpy? That's a bit more involved and we can get into it, if that is the issue.