Are there checks like: certain speed has to be gained at halfway down the runway or v1 has to be reached at a certain point (down the runway)?
There were some runways, perhaps military some years ago, that had 'distance to go' markers. You could monitor your speed passing certain markers and make a rough guesstimation of 'how's it going?' It has been discussed before for modern civil airliners, but it was deemed the RTOW analysis was good enough.
How you would be supposed to notice you're not meeting this net performance requirement, I unfortunately don't know.
One day in Mombasa, B757, very hot. We did all the calls and arrived at a conservative thrust/flap setting. As we trundled down the runway the end seemed to be coming closer very quickly. A manual nudge of the TL's to the stops seemed appropriate. We surged forward and rose skywards; as you do. While this was happening, we crested a gentle hop in the runway ands realised we'd been fooled by a mirage. When there's doubt there is not doubt. Stopping was not a safe option as everything else seemed to be working fine and we had no doubt about the calls: it was just our eyes were being deceived.
Let's just say that if you want your aircraft to take off you're gonna have to put the nose up at some point.
Is A340 one of these clever birds that weighs itself and calculates the trim? If the trim was a little nose heavy PF could have been applying up elevator very gently and been surprised that it wasn't working as expected; then a more muscular pull delayed the lift off point.