You won't, It's a surprise!
Any Hatchimal you hatch in Hatchtopia Life will be random, just like the Plush Toys.
If you scan a code from a Hatchtopia Life Plush Toy, your digital hatchimal will be the same rarity as your Plush Toy (e.g. if your Plush Toy is Rainbow Rare, so will be the Hatchimal you hatch in game)
Hatchimals hatched from Mystery Eggs could be any rarity!
Appearance Rates:
Common Hatchimal: 70%
Rare Hatchimal: 27%
Epic Hatchimal: 3%