If you're working on older airplanes, or possibly some homebuilts, you might come across a fitting that doesn't look quite right. Is it an automotive 45 degree flare or is it an aircraft 37 degree flare?
37 degree or 45 degree?
Here's a quick method:
Draw a right angle onto a piece of paper and place the fitting onto the paper as shown below.
45 degree flare
45 and 45 makes for a right angle (90 degrees)
37 degree aircraft AN or industrial JIC flare
If you assemble your own aircraft hoses using hose mandrels you might find my article: Aircraft Hose Assembly - Battle of the Bulge of some interest.