There is really no way to determine visually carbonation. SOmetimes folks see krausens on top but most of the time not...the only way to insure carbonation, is to give the beer enough time to carbonate. That's it...If you added sugar, you beer will carb when it's time.
The 3 weeks at 70 degrees
, that we recommend is the minimum
time it takes for average gravity beers to carbonate and condition. Higher grav beers take longer.
Stouts and porters have taken me between 6 and 8 weeks to carb up..I have a 1.090 Belgian strong that took three months to carb up.
Temp and gravity are the two factors that contribute to the time it takes to carb beer. But if a beer's not ready yet, or seems low carbed, and you added the right amount of sugar to it, then it's not stalled, it's just not time yet.
Everything you need to know about carbing and conditioning, can be found here Of Patience and Bottle Conditioning.
With emphasis on the word, "patience."