How Do You Go About Checking On The State Of Your Child’s Nutrition?

We have a 5-year-old and an 18-month old and we are all Vegan and while they are very healthy and strong and we take their nutrition very seriously, we are curious to know how you go about finding out how they are doing and if are getting enough calcium, protein etc.
Is there a test you can do to find this out? If so, how much is it and where can you get it done?
Cheers