One Space Shuttle was destroyed due to failure of the insulation tiles. SpaceX is reportedly using another technology, which replicates sweating , for wearing off the reentry heat. How is the effectiveness or efficiency of these reentry heat shields best simulated or tested on Earth?
Ground testing of heatshield components and materials can and has been done in arcjet facilities.
An arcjet typically combines a super- or hyper- sonic wind tunnel with a large electrical heating device. "The gases (typically air) pass through a nozzle aimed at a test sample in vacuum, and flow over it, producing a reasonable approximation of the surface temperature and pressure and the gas enthalpy found in a high velocity, supersonic flow of the kind experienced by a vehicle on atmospheric entry."
(sourced from first link)
NASA Ames has four arcjet facilities in its arcjet complex. "The largest power supply can deliver 75 MW for 30 minute duration or 150 MW for 15 second duration."
Johnson Space Center also had an arcjet facility that was last used heavily during development of shuttle tile repair materials and procedures after the Columbia failure. It was closed in 2014 due to redundancy with the Ames facility.
The first link in this answer is a very good overview of arcjet testing and facilities, please see it for further reading.
Section 4 of the document "Test Planning Guide for NASA Ames Research Center Arc Jet Complex and Range Complex" contains a detailed description of the arcjet facilities at Ames.
To test a heatshield properly, you build a test article and reenter with it.
To test it adequately, you put it in a low pressure wind tunnel, and blast it with plasma blowtorches. Example Ames research Arcjet facility testing SpaceX's PICA heatshield material for the Dragon capsule.
P.s. The idea of transpiration-cooled heatshields is out of date. There is great uncertainty about just what SpaceX is actually going to use as heatshield, and how it is affixed to the Starship. Recent test Hoppers have had some sort of hexagonal tiles affixed to them, but these have not survived the launch or even static fire tests very well. Long article about the Starship heatshield tiles on Teslarati's website.