It is placed at the point in the queue where it splits between general standby queue and the backwards \ fast track \ single rider. The amount of space in the queue before that split isn't viable, and on any normal operating day, standby shouldn't extend beyond that point so everyone can try it before joining the queue. The only times i've ever seen standby exceed that is when half of the standby queue is closed and cast aren't on their game.
In my experience, tester seats aren't always that reliable - I can fit the claw tester but can't on the ride, and vice versa with rivals - rivals tester says no, but the ride says yes.
Ride operators will do everything they can to make sure you fit within the required parameters, provided you can work with them and work quickly. Different rides have different tricks if you're 'on the borderline' to get things to lock - on Rivals, you might need to lift yourself up in the seat by pressing down on the outside bars of the harness. On scooby, "spare tyres" may need to be lifted out of the way to get the harness to sit low across your waist. Scooby has more room in one row versus the other ( @Spotty help me out here) and the legroom aspect helps keep your thighs low enough to get it to lock as well.
If you're concerned, tell the operator \ grouper and they'll make sure to check on you, and help you with any tricks. If they can't get it to work they'll unlock and apologise and direct you to the exit. There is no shame in trying and better to give it a go and know than avoid it 'in case i don't fit' and miss out on all the fun.