Matt,

Here's a trick for you. When you have a model ready to cover but before the dope. Give it a "bath", wipe it down with a wet (not dripping) cloth. This will raise all the little dents and places you pressed too hard when you sanded it. Let it dry and give it a once over with 400 grit paper in your hand, no block, just before you start doping. Another thing I do when I get to the finalization point is to spread an old wool blanket on my bench and move all the hard items that can poke to wingtips and such. I do all my fine finish sanding on that old blanket, but I still "bathe" the model prior to dope.

Candling realy works but it takes some practice to get good at it, you have to inspect every square inch. YOu'll find that the big open areas are usually pretty good. Its the areas near edges where we tend to under-sand trying not to cut too deed. Candling works better for primed surfaces but will work on wood as well, I just never got into it at that point.

Very careful selection of the balsa and twice as much care as you built really minimize those little dents, but even your fingernails and edges of sandpaper and sanding blocks can cause them.

I need to add my two cents worth here Matt.
I NEVER sand anything flat without some form of block. It is way to easy to cut finger grooves in it. Even using a foam sanding pad is better than your fingers EXCEPT over open bays .
Otherwise, all good advice.
There is also another way to judge how flat and perfect your surface is. i use two products, I use them on my restoration projets, stuff like 55 Thunderbirds, 60s camaros,, and other hot rod projects. One is a spray can that is called Guide coat. It is a very thin black spray paint that you lightly dust (spray) on the surface, then when you sand, the low spots be they small or large, will stay black as you sand the high spots down. MUCH more visual than candling. The other product works the same way, but its a powdered guide coat that you put on. Its like powdered chalk that you dust on the surface and as you sand, same thing, it stays in the low spots and sands away from the high spots. The spray is about 6 $ a can and should be enough to do several planes, it goes on light ( shake it THOUROUGHLY) the powder is a bit more expensive,, like 20$ for a container that includes an applicator, it will probably be enough to do everyplane you ever build even if you build like Jack Sheaks! ( or Randy powell,,, ) You can get the same effect if you want to try it without the investment, find some black charcoal, like artist charcoal, pick a hard grade,and sand it on a peice of 320 or 220 sandpaper, then using something like a small piece of foam rubber, or sponge, apply the dust on the surface , rub it around so that the whole surface has at least a grey color, then start sanding. I would advise against this if you plan on a transparant finish, but other than that,  a light wiping with a tack rag, or a can of compressed air will get rid of any loose powder before you start doping.