If you have access to another person, that person can hold bunny in his lap and make bunny sort of sit on its butt facing you. You don’t have to make bunny lie on its back to see the private parts. The other peron supports bunny against his body and holds up the “torso” with one hand around the chest. The other hand can go under the bunny butt and hind legs for extra support, you want to avoid all kicking backwards. This is probably not possible with a Flemish Giant, only with smaller buns.
Another possibility is to have quick look while your bunny is periscoping to reach a craisin. That would only let you spot real trouble, nothing subtle. You can also place your bunny on a mirror and try to get a view of his underside in the mirror.
I’ve one bunny who won’t have anyone touch his belly and one bunny who likes being pet on his tummy. I guess it’s normal either way but more common that they are reluctant to have their underside checked out. There is a reason why there is an underside, i e, theres no bony protection around the inner organs there so that area is more vulnerable to attacks.