For longitudinal levelling, I used a DIY store laser level mounted on a camera tripod and shone it on the longeron rivet line. Hard to beat a 10 to 12 foot level line.
FYI, I had high confidence in that laser. When doing my gear fairings, I pretty much used vans instructions but I included the laser as a tool. At one point I shined the laser onto the plumb bob line. It was kinda cool to see the line flare into a brilliantly illuminated vertical line.
For levelling in the roll axis, I used a good quality bubble level and used the proper techniques (flipping it 180 deg and averaging bubble position) to account for manufacturing error in the level itself.