Tom Kirkman pointed out that my earlier listing didn't seem quite right - and he was right. I listed a conversion from grains to grams, but accidently mentioned "oz" instead of "grams". So the correct approach is:

Weigh the 1st 30ft of tip section on a scale that weighs in grams (or oz., but weighing is grams is easier) (small kitchen scale). Fly line weights (in grains) are:
3wt=100+/-6=(about) 6.48 grams
4wt=120+/-6=(about) 7.78 grams
5wt=140+/-6=(about) 9.07 grams
6wt=160+/-8=(about) 10.42 grams
7wt=185+/-8=(about) 11.99 grams
8wt=210+/-8=(about) 13.61 grams
Covert grains to grams. The following web site provides a calculator to make the conversion easy:
[www.itinscales.com]

Now, even though the "official" line wt measuring method ignores the line's tip, just weigh the 1st 30 ft, including the very tip end. For the purposes of trying to figure out what line you have on your reel, the difference in weight between a 3wt and a 4wt, or any other wt (including the tip end), is big enough, when your weighing in grains/grams, that you will be able to tell the difference. (If you try to weigh in ounces, the scale may not be sensitive enough to catch the difference.)

When I was faced with the problem of having a line on my reel that I knew was either a 4wt, a 5wt or a 6wt (because I had lost one of the 3 and didn't know which was which of the remaining 2), I actually bought a kitchen scale that weighed in grams. I coiled up 30 ft of line and laid the coil on the scale-with the remaining part of the line held up off the scale. Or course this did not give me a "really really" accurate measure of the line But it was sure accurate enough to tell me that I was weighing my 5wt line and not the 6wt line. Furthermore, the weight that I registered was well within the 5wt weight-range specification of 140+/-6 grains.

I read somewhere - and if I could have remembered I would have save myself the cost of the kitchen scale -that someone set up a balance arm scale from a coat hanger and had pre-weighed nuts (the bolt kind) and washers that he would hang on one side of the scale arm and hang his 30 ft of coiled line on the other arm and when the scale was balanced (horizontal), he'd count the nuts and washer and know what line he was weighing, because he knew what those nuts and washers weighed. There was a little more to this scheme but that is all that I can remember about it; I think that the article gave some weights of various sizes of standard nuts and washers so that you could use the technique without having to go out and select/weigh nuts and washers yourself.

When I was trying my experiments with weighing lines, my problem may have been that the scale was too sensitive. Though rated at .1 grain, with proper calibration and good light, it's possible to read to .025 grains. A single #28 hook dropped on the pan will deflect the beam easilly. The slightest puff of breath or movement would set the beam swinging wildly. The difference in line wts, till the heavy ones, is about the equivalent of 2 or 3 BB size split shot; so that should be measurable enough to tell the difference between one weight and another, gotta try it with a different scale.

Another thing that is helpful is to mark your lines when you get them. I take a permanent magic marker and make the appropriate number of marking rings around the line near where the leader is attached so I will always know what I have on there. A 4 weight gets 4 rings or bands, a 5 weight gets 5 and so on.