Good question: you are correct that there are clearer are more widely applied guidelines on tree health metrics for larger trees (for example in an urban forestry context).

The most common health metrics used for saplings are essentially based on growth as an integrated measure of tree health, with non-destructive measures commonly based on root collar diameter and height (and sometimes leaf area).  Annual extension growth quantified by terminal leader length is also useful, particularly in temperate or boreal trees with annual internode production.  One can also utilize either sequential harvests or allometric estimates to derive "growth analysis" metrics (i.e., relative growth rate, leaf area ratio, unit leaf rate (= net assimilation rate), leaf mass fraction, etc.) that have been commonly used in the agronomic literature (Hunt et al. 2002 give a helpful overview).

Depending on the context (and available instrumentation) additional useful metrics would include leaf survivorship, leader loss, leaf chlorophyll content, chlorophyll fluorescence parameters (especially Fv/Fm), and photosynthetic gas-exchange parameters.  There is of course a large plant stress physiology literature and many other techniques that could in theory be used (membrane leakage, antioxidant content, analysis of xanthophylll cycle pigments come to mind), but most likely these measurements are mainly useful if you are seeking to understand physiological mechanisms involved.

I've attached a study from my lab as an example that includes parallel work on canopy trees and saplings.

Reference:

Hunt, R., Causton, D. R., Shipley, B., & Askew, A. P. (2002). A modern tool for classical plant growth analysis. Annals of Botany, 90(4), 485-488.

Article Responses of Acer saccharum canopy trees and saplings to P, ...

Tree health can be measured with the following parameters:-

Tree height, diameter at breast height, bole height (height to the first branch), crown diameter, number of main branches, branch angle, straightness, leaf color as well as production of fruits.

Tree health can be measured with the following parameters:-

Tree height, diameter at breast height, bole height (height to the first branch), crown diameter, number of main branches, branch angle, straightness, leaf color as well as production of fruits.

Tree health can be measured with the following parameters:-

Tree height, diameter at breast height, bole height (height to the first branch), crown diameter, number of main branches, branch angle, straightness, leaf color as well as production of fruits.