For just the pad and leveling the dirt, I just tape my level to a 12 foot long board that is perfectly straight and set it on the pad. I decide where my high spot is and work to build up the rest of the pad to that point. Most of the time I can pretty much eyeball it and get close, so the level on the board is for fine tuning and figuring out where those low spots are that will take up extra concrete when it's poured.
For the forms, I make sure every board is perfectly straight, then I set them one at a time and make sure they are level. If I do it right, by the time I get back to where I started, the last board lines up with the first one!!!1
I bought a used Spectra Laser level on ebay quite a few years ago that works great for really big areas. It was very useful in building the dam on my pond so I knew where the shoreline was all the way around it. I rarely use it for smaller projects, but did one time when I didn't realize my six foot level was off. Those forms came out over an inch off and I used the laser level to redo it. It's easier and faster if you have a really good level to set forms then messing with the laser and the stick.
On my gazebo, I did most of the pads with a straight board and a level, but for the longer spans, I bought a length of clear tubing and filled it with water and stretched it between the pads. It was also good for double checking the other pads that where done with the straight board and level.
Whatever you do, try to double check it when it's done with another method.
Eddie