Honestly the easiest way is to use a bathroom scale but it sounds like that wasn’t very accurate for you. Feedback Sports makes a handy scale just for bikes but it’s pricey. Only true weight weenies will spring for one of these. 🙂
Weight is at least few notches down the list in terms of important attributes for any bike that I will own. Handling/geometry, suspension/pedaling performance (if it’s a full suspension), durability come first for me. Last year I demoed a highly regarded bike on a familiar trail back to back with my own bike (BTW, I think this is the absolute best way to do any demo). Visually and geometry-wise the bikes are nearly identical with no more than a half a degree or a few millimeters difference on only a couple of their measurements. The demo bike retailed for over $10K and mine would be around $5K (if I had paid retail, but thats for schmucks and Christmas shoppers; Happy Holidaze!). The demo bike was also exactly three pounds less than mine. Riding wise they were the same on all portions of the trail except one, the climb. My heavier and less expensive bike climbed better and faster. I attribute this to it’s superior pedaling ability and maybe my own familiarity with it. That said, I know how to set up suspension as well as fit and three pounds is a noticeable difference. It isn’t a difference that I’d be willing to pay for and certainly not at $5K for three pounds. So to the OP, you can save all kinds of weight by whipping out your credit card. Do so if it makes sense and turns you on.
I don’t really obsess over the weight of my bike. While I don’t want to feel like I’m towing a battleship, in my opinion durability is more important than being lightweight. I’d rather have heavier tires that don’t roll as fast but are also not consistently bent or out of true. This could be because I don’t race or ride competitively. If I was going to cut weight, I would probably start with new handle bars and a new seat/post. I recently upgraded my cassette and was amazed at the weight difference between the old cassette (heavier, yet less gears) and the new (lighter, more gears). I was amazed at how these seemingly weightless parts added so much to the total weight of the bike. I thought that most of the weight that matters at least is in the frame, suspension, and tires/wheels.
Luggage scales work just fine and can be “had” for $20. If you buy one with a medal hook, just add a little duct tape, so it doesn’t scratch your bike. And if you travel frequently, of course, the scale can double up for your travel purposes.
Not a weight weenie. Both my 29ers weigh in the 26lb range. One is a Cannondale Carbon Trigger and the other is an alloy Santa Cruz Highball. I also thought my hardtail would be a couple pounds less, but apparently not despite my Trigger having a bulky rear shock and a dropper and my hardtail having a carbon seatpost they weigh the same, my hardtail will actually weigh more once I put my new grippier tires on it.