[RPG] On Roll20, is it possible to fill in character sheets without being in a game

I allow them to reconstruct it during play. To a degree, this depends on the game, but even in the more complex games it's the end results that matter more than the formulas. For example, if I know that my energy blaster in Champions has a big energy blast with a bunch of different options, I can probably get away with saying I do 12d6 with a single-target blast and maybe 8d6 with an area effect blast. I don't need to reverse-engineer the points I spent on each version of the power.

Likewise, if I'm playing 4e, does it matter that much if my daily does 4d8 or 3d8 for one session? Probably not.

I also don't need to figure any of this out until it comes into play. Did my Unknown Armies character have 55% or 65% in Running From the Fuzz? Until there are cops on my tail, I don't need to even think about it. I don't want to burn an hour of everyone's time asking about the details they remember before we sit down to play.

Here's how I've always understood the sheet:

The Modifier space is where you put the Ability Modifer - so with a STR of 14, you would add +2 (page 36 in both "Heroes of.." books.) This box is helpful for powers which have a secondary effect. For example - one attack might also give allies X Temporary Hit points, where X is equal to a certain Ability Modifier.

As mentioned in the other answers, the "Check" space is where you put your TOTAL. This is the value you would use for an Ability Check (using raw STR to open a door), or a Skill Check (Using STR-based Athletics to scale a wall).

The Misc. space is where I usually put special modifiers. Usually this is +5 for a trained skill, +2 for a Racial or Feat Bonus, or a -1/-2 for an Armor Penalty. I don't usually write down the half-your-level math since that is always a part of both checks.

For the Weapon/Powers, if you don't have a DDI and aren't using the online Character Builder - you'll probably want to use those for your most common attacks. Generally, I tend to fall back to writing the actual calculations on the back of my sheet and just keep the front of the sheet clean and readable so it's easy to refer to in game.