[subcool]

https://www.youtube.com/watch?v=4IkSaMq8SgI&feature=youtube_gdata_player

 

[beenthere]

I can't tell much by the flames. I've seen flames like that produce low amounts of CO, and high amounts. Need a combustion analyzer to know how good it is or isn't burning.

 

[techpappy]

Flames look good but, as Beenthere said, you can't really tell without using a flue gas analyzer. However, I would be more concerned if the flames were burning lazily/orange/ color.

 

[hvac benny]

I've seen perfectly blue flames that turned out to be 20,000 btuh's over fired (gas appliances are not permitted to be over-fired, but can be under-fired by up to 10%). As a DIYer you aren't going to buy a combustion analyzer, but you could clock the input of your furnace at the gas meter to determine input.