Your question is not at all clear. However I will try to answer based on the information you supplied. Your power supply is rated to supply 30 kV at 10 ma. That is 300 watts so I assume your supply is pretty large and heavy and that your load needs that much power. That said, you also stated that you don't know the resistance of the load you plan to connect to it. That is not good. How do you know your load will not try to draw more than the supply's rating of 10 ma? In that case, your supply is not adequate for your application. However, putting that aside, to test the supply at full power requires a resistance of 30 kV/10 ma or 3 megohms. This resistance must be able to simultaneously withstand 30 kV and 300 watts. That will probably require, as you said, a series string of resistors. How many depends on the individual ratings of each resistor. Resistors that can withstand high voltage are readily available (I have a relatively small 100 megohm resistor rated for 20 kV). The problem will be to get the combined rating of 30 kV (probably at least 50 kV for safety) and 300 watts (again increase this to at lest 500 watts for safety). To measure the actual output voltage will also require a voltage divider but this could consist of several high megohm low power resistors to minimize the power drain. Assuming this is all accomplished, you can measure your supply's output voltage with and without the load to see how much voltage drop there is. Again, though, since you don't know what your actual load is, this information may not be all that helpful.In closing, I will say you should exercise extreme caution when using this supply for your safety.