@Hale
Thank you. Finally, someone is actually trying to answer my question rather than assume what I’m talking about or pass judgement.
Yes, you will skip the weight check if you already have a boarding pass. I do this all the time which is how I get through in 15min (internationally). I travel with hand luggage only.
Low-cost airlines often check at the gate and will gate-check. If luggage isn’t free - you will be charged for it and it often costs more than if you bought luggage allowance ahead of time. I’ve seen this with Ryanair, EasyJet, JetStar, Avianca, and Bolivia’s main airline.
I still use this all the time because:
- They don’t always check
- Gate check means it’s more likely my bag arrives (I always have the essentials in a bag I take out of the main bag)
- I skip the awful long line to bag drop.
I got gate-checked with Air India this year, and I saw many in South America (LATAM) having to do this.
My bag looks a lot smaller than it is - when I carry it in “portfolio mode.” I use vacuum packs to make it look even smaller. It has never been checked. This is the secret trick, imo.
@Tan
Unfortunately, I don’t have this ability to use your secret trick if I’m understanding it correctly. 
When I measured the dimensions and really smushed it down as much as possible, the bag was about 20.5 x 15 x 8 including wheels, handles, etc. Slightly too wide for American flights but about perfect for everywhere else. I have no idea if they would find this suspect, but it is visibly within the dimensional limit. The only issue is the weight being 2.5-3 pounds heavier than other suitcases. My hands were kinda tied since I’m trying to get a non-American-sized suitcase as an American. I basically only found one suitcase which was from a company that I trust and was the right size with two wheels.
If you travel carry-on only, sometimes you’re able to bypass the check-in counter and go straight to your gate. However, once you get to your gate, you’re at the gate agent’s whims.
Some airlines will give you a tag saying the weight and size have been checked at the check-in counter and you’re given a tag to state it’s within the limits. Without this tag, you could be asked by the gate agent to have your bag sized and weighed.
Some airlines at some airports will not allow you to use an electronic boarding pass, which means you’ll be required to go to the check-in counter to get one, where they will likely weigh and size your bag.
The only way to ensure you get your bag on that flight without having to pay the extra fees is to be within the weight and size limit allocated to you according to the fare you paid.