Measuring the fork rake is not that hard, but you also need to consider the fork length. If the frame is a horizontal TT model, you'll get some idea of incorrect length if the TT is not level after being assembled, but that's kind of late to figure out that you have a problem. The wrong fork length will change the HTA and cause as much or more change to the steering trail as the wrong rake.

A common method of measuring rake with the headset installed would be to measure the front-center with the fork in it's normal position, then rotate 180 degrees and measure the difference. Divide by two to get the rake.

If you have a local machine shop and tell them you want to measure the distance between a line through the steering axis and a parallel line through the center of the axle, they'd have no problem setting that up. It can even be done with reasonable accuracy on a flat countertop with a precision scale and a smooth board of uniform thickness to hold the fork crown off the countertop, but you have to know what you're doing.