It's not just bass players on Talkbass who are concerned about weight, many guitarists are a bit fussy about the weight of Strat and Tele bodies and most 6 stringers are way lighter than your average bass.
I've always had a set of accurate scales on hand as I build solid body instruments, my current workshop digital scale cost me twenty dollars a few years ago and it's very accurate. I check it regularly by repeatedly weighing an old set of scale weights. These days a reliable set of digital scales that will weigh up to 5 kilos can be bought for ten bucks.
Rich is right, no one would be happy about buying a bass and finding that the color was wrongly described, or the pickups weren't what was claimed, or it had way more dents and scrapes that described, so why should dodgy weight claims be OK?
If it's a bass costing several hundred dollars, or several thousand, take it somewhere like a post office (maybe your local deli, they have scales that are checked regularly) and get it weighed, I'd even take a photo of the bass on the scale with the weight showing.