The first thing I do when I walk onto a new job is sight-down the stem walls. On those few occasions when they"re not straight and flat, I pull the lines square and smooth the bumps before I even think about framing. Spend some time setting your sills correctly, and the rest of your framing can fly.
Start at the Stem Walls. Check your stem walls for square by pulling a 3-4-5 triangle. Determine the two longest axes of the building. Where these axes intersect, hook your tape on the corner and pull it 3 feet on the shorter of the two walls. Mark 3 feet on the outside edge of the concrete. Next, measure 4 feet on the longer wall. Hold your pencil against your tape at the 4-foot point and use it as a pendulum to make a line 2 or 3 inches long from the outside edge of the concrete arching inward.
Now, measure from the 3-foot mark on the short wall to the 4-foot line on the longer wall. Mark where 5 feet (on your tape) meets the line you drew at 4 feet. This mark represents a right angle and is square. Ideally, the 4-foot and 5-foot marks will meet at the outside edge of the stem wall.
Now that you have a dependable (but small) right angle, you can extend this line two ways. You can pull a string line from the corner of the stem wall through the point where the 5-foot line meets the 4-foot line, carrying it straight down the stem wall. Or (my preferred method) you can make the largest triangle possible, e.g. 6-8-10 or 30-40-50; the principle works with any multiple of 3-4-5. And, as a double check, I often pull diagonals, but use the primary reference lines the 3-4-5 establishes. Everything else from room/wall dimensions to stud/joist/rafter placement can be pulled from these two primary lines.
Adjusting the Square. If the stem walls are out of square, you must adjust both lines of the right triangle in or out from the edge of the concrete to keep the mudsill–and the floor system you build on it–square. By turning the triangle, you can move the reference lines in or out along the edge of the concrete. Always move your reference points to make the placement of your sill as flush as possible with the concrete to avoid problems later with sheathing or siding. Err on the side of having the sill overhang the concrete so that sheathing will be able to drop below the concrete, otherwise you"re cutting sheathing at the concrete.
If the stem walls are severely out of square and your sill plate just won"t align with the edge of the concrete, inform the general contractor or concrete sub before proceeding. Either of them might choose to correct the problem themselves. (A word to the wise: No one likes to be told that their work is wrong. Check, double-check, and triple-check before raising a red flag; you"ll be required to prove your point. If you"re right, hold your ground. Never agree to do your work wrong in order to make someone else"s look right.)