To measure fork rake, there are 2 easy ways, the first, quoted from a bike geometry compilation project, but if you like life to be simple, skip to the other second method.
"Front-end geometry is the hard part. Here I like to put the bike on the ground with the front wheel on a piece of white paper, sitting as level as possible and blocked so it does not roll - little pieces of wood again. The front wheel contact is vertically under the QR skewer, so you can mark the contact point on the paper. I use a yardstick (aluminum for stability) to project the head tube/quill center line down to the floor, then I need an assistant (fellow cyclist/geometry dweeb is best, wife is second best, cat is worst) to mark the contact point. Then you can measure trail directly. Armed with math skills, the head tube angle, the trail, and a calculator, you can compute fork offset. You can also measure it directly from the yardstick with a second ruler, if everything is held steady and your eyeball is good, to approximate the perpendicular distance. If the fork is off the bike, you can block the steer tube so its level on a level table, then measure the height of the steer tube to the tabletop and the height of the dropout center to the tabletop. This is a direct measurement of the fork offset."

I run a string - you need quality string, not the cheap twisted frazzly stuff that is usd for tying small parcels. I found an old spool of sew-up tires thread. It's thin enough for precise measurements, strong enough to be strung without braking, and flexible enough for nice straight lines.
With the front sheel off, run the string through the front dropouts, and up over the steering stem extention. If need be, put a piece of handlebar tape or masking tape or electrician's tape over the stem extention (something that will keep the thread from slipping). NOt over the thread, but under it. If there is a slight slant to your stem, slide the thread to where it is shortest and tie your knot.
Then, adjust the thread so that it is parallel to the headtube. With sight, all the threads must be parallel, co-planar - in cluding the segment that runs through the dropouts. (for the readers who may now know what co-planar is, it means they're all in a same plane... in other words, if you had a plate of glass up against the threads, none of then would have any extremity lifting off of it, they would all be paralell to it in all directions.
I hope you have a vernier caliper, because it's the easiest way. But a tape measure can do as well.
Now just place your vernier caliper to measure the distance from behind the steering column to the threads, placing it at a height where you can get both threads behind the jaw for good alignment. Visually make sure that your vernier is perpendicular to the head tube. Take your reading. Measure the diameter of the head tube (I suppose if you're using a tape measure you can always look up what that diameter would be - not to be confused with the diameter of the steering column that goes through the inside of it as that one would be 1-1/8 but that's not what you want). Divide the head tube diameter by 2. Subtract it from your distance, and you have your fork rake. See, it's easy... the hardest part was reading this all.