The safest choice is to bit something on everyone.
You can open the text panel where Nazuna tells you about each racer, and their chances of winning.
This is the core 80% of truth about the race. The remaining 20% is pure RnG as during the races the track generates random obstacles that can slow or even knock back any of the racers.
For example a racer who has a parameter written - She is stubborn and works very hard the closer to the finish line - means something like - she goes faster when you get to the last part of the race. Ive seen some girls who got knocked back to the last place, go Turbo mode and end up first or second. So reading up on each of the racers can help you get a good idea of how the races is gonna progress, before the RnG is factored in.
The problem is that with the delayed launch of the race, we dont have as many bidding currency as we would have by this point, were it implemented from the get go when the event started, thus its very hard to bid high enough sum on each of the girls, like I would do in the DMM races, where it doesnt matter since I have so much of it laying around.
To go safe just bet a little something something on everyone, or if you feel adventurous read on the racers, and bid on the ones who seem to be the best candidates for Victory.