How do I know if the substrate is sufficiently strong?

A solid substrate is primarily identified by the fact that it does not crumble, dust, peel and does not deform under load. You can use a tap test, scratching with a spatula, and a simple impact test as a rough guide. If the substrate is easily damaged even before application, microcement will not magically turn it into a concrete slab. In such cases, it is better to strengthen the substrate, apply a suitable levelling compound or use an epoxy system with sand. Conducting a preliminary impact resistance test is advisable: drop a common object from a height of about 1 metre and observe the extent of substrate damage. The putty will not fundamentally improve this property of the substrate. If the substrate is of poorer quality, an epoxy primer with silica sand can be used, or the surface can be covered with a quality levelling screed with the required parameters.