The easiest way to measure air leakage is with a blower door.

However, one doesn't need to invest in that much technology, nor achieve a specific depressurization, in order to guide the air sealing work. In my experience, it is essential to be able to check the effectiveness of caulking or foaming periodically as you work. In theory, you can identify leaky spots, seal them up, then retest with a blower door. In practice, many of those leaks don't get sealed up even when you think you're hitting the right spot with the spay foam. It can take several tries to really stop the draft. So, it makes sense to set up a fan that can be used from the beginning to the end of the air sealing process. If you have access to an affordable blower door for that length of time, then great. If not, anything that will induce a draft will do.

Smoke Pencil is indeed the best device detect the draft and air leaks at your home. It is now used largely even by professional Energy Raters and HVAC installers because of the efficiency and safety it has in detecting the drafts. You can Google it and can buy online.

Absolute Steel—Steel building kits easily assembled by unskilled labor using common tools. Watch the video & see for yourself.