Courtesy of u/aepurniet from this Reddit post.
The statement must be prefixed to the statement with some value definitions, then parse and then type checked. The resultant type of the expression can be checked from checked.tpe.
def main(args: Array[String]): Unit = {
import scala.reflect.runtime._
import scala.tools.reflect.ToolBox
val toolbox = currentMirror.mkToolBox()
val expr = "age > 18"
val eval = s"""
|val age: Int = ???
|$expr
|""".stripMargin
val tree = toolbox.parse(eval)
val checked = toolbox.typecheck(tree)
println(checked.tpe)
}