Get three or four large wattage resistors in the 100 watt or better class for the lower values. You want around, 50 k, 25 k, 12k and 5 or 6k, even a 2 or 3 k if you find one that can handle high current for a minute or so.
Place them across the HV winding measuring a voltage for each resistance. You do have A B&K vom that has a 5 kv scale don't you?
Don't worry about measuring current. That you can calculate that at any time.
Simply Plot the resulting values, Resistance vs. voltage on a sheet of semi-log paper.
You will see a straight line, ( such and such constant per decade or octave. )Any points along that line indicate the transformer can deliver.
When you see a roll-off or fall-off towards the line at low resistance, high current values, you will have found the limits of what that transformer can do cold. Go back to the last approx. medium point on your line and assuming that resistor can take the heat, run it for an hour or so and measure temperature rise on the transformer. This will give you a reasonable indication of what the transformer can do in IVS, almost ICAS conditions, actually a little better. --25% down from the lightest load point will be a better approximation for IVS. The mid-point will better represent a 10 minute AM qso. , provided you listen for the following ten.
In my case I have a 24k center tapped, 225 watt resistor that I used as 24 k all the way down to 6k by using one or paralleling both 12k halves. This gave me confidence to use a 800-0-800 vac transformer rated at 800 ma in a full wave bridge circuit for a pair of 8875's IVS.
You should have seen the same resistor points on my 3kv/1amp CCS p. Dahl. Hint , the log decade slope was not at the same angle and no, none, nada roll-off observed
