How can you tell if a toddler is truly lactose intolerant?

Try Lactaid milk. If suspecting lactose intolerance, a parent can feed a toddler or child lactaid lactose-free milk for 2 weeks, and then "normal" milk for 2 weeks, and then lactaid milk for 2 weeks, and finally go back to "normal" milk. If the child has gas, bloating, +/- tummy aches when on "normal" milk, but not on lactaid milk, then the child is very likely lactose intolerant.