You're looking for signs of it causing some mushy or watery cecotropes, or upset stomach that results in minor reduced appetite. If you notice any of these things, stop feeding the veg immediately and closely monitor bun making sure everything gets back to normal within that day.
If it's been more than a few days since the bun has been off of the greens that she's used to, probably best to start off with a small amount the first couple of days, then if all seems normal the amount can be gradually increased. If in doubt or concerned about bun being sensitive to greens in the diet at this stage, you're better holding off until bun is a bit older. Though usually a baby bun that has grown up eating veg with mom, won't have problems continuing to eat them.
More severe reactions to look out for are severe mushy or watery poop, where the fecal balls are also pudding like or watery, which is a much more serious digestive issue and is usually an emergency due to the risks of severe bacterial enteritis. If an upset stomach occurs and causes bun to stop eating, baby gas drops can sometimes help, but if bun isn't back to normal in a few hours, best to get to a rabbit vet. Or if you notice signs of bloat, get bun to an experienced rabbit vet immediately.
https://bunssb.org/bunnies/guide-bunny-poops/
https://wabbitwiki.com/wiki/Gastrointestinal_stasis
https://wagwalking.com/rabbit/condition/distension-of-the-stomach-with-gas-and-fluid
It can be a bit complicated with a new bun and figuring out the right diet. Generally, always introduce new foods one at a time and slowly, with the exception of grass hays(almost always well tolerated). And the best way to spot if there's a problem is by closely monitoring a rabbits droppings and food intake, as well as weight and body condition. Any change in a rabbits poop or change of appetite, can signal there's some sort of health issue developing, or if a bun is feeling boney and losing weight.
Monitoring your rabbits weight
http://www.medirabbit.com/EN/Food/Food_main.htm
https://wabbitwiki.com/wiki/Veterinary_emergencies
https://rabbit.org/veterinarians/