I'll add solder on the opposite side to which I apply the heat as most situations allow. Where you can't do that you just know by experience that you've done it right anyway.
I am often amazed at the song and dance some apprentices (and a few plumbers) make when soldering small bore fittings. They wave the torch around, move around the fitting changing their stance, adding too much solder then going back at it with the blowlamp and adding a bit more solder then it goes black. I forgot to say they've put so much flux on the pipe and in the fitting (don't flux the fittings!) that there is flux pouring out and dripping on the floor followed by blobs of solder.
I have to unteach and retrain them so often. It's possible to teach an 8 year old child to solder properly so I don't understand why people make such a meal of it.
Lastly my pet hate is watching people add more solder when they are using a Yorkshire fitting. If you are doing that then why even bother spending more money on Yorkshires when you are endfeeding anyway? It makes no sense.