From the video linked, it sounds like they can (legally) switch bikes during the race, so they could just switch back to a legit bike close to the finish.
Even with a big lead you would be hard-pressed to have a mechanical requiring a bike change and still win. Yes you can switch bikes but you will be trailing the peloton by then (where the support cars are.) Early or mid-race you could catch up to the peloton and possibly make it back up front with a lot of effort but probably not within the last 20km or more.
Cut up all the bikes. Or use the more reasonable suggestion in the sibling to my comment and just dismantle them enough to do a good inspection.
Or, you know, just X-Ray them. Aren't most competition-level bikes using carbon fibre frames these days? For metal frames, ultrasound would work just as well.
X-Ray would work on the thing aluminum chassis of a low end racing bike too. You might have to crank the power up a little, but it's going to see those powerful little magnets clear as day.