There's a technique described HERE which involves measuring the battery voltage off-load and then on-load. From the voltage measurements and the load resistor you can calculate the internal resistance, although this is an approximation as the internal resistance is not linear with current drawn.Don't try what THIS person did to win their Darwin award. (For the uninitiated, the Darwin awards are for those people who do humanity a favour by removing their genes from the pool by killing themselves in particularly foolish ways, thus reducing the net stupidity of the human race.)
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It depends on the load you connect, the battery chemicals, charge and temperature (the internal resistance will increase when the battery gets hot).
But you can find out the internal resistance of a 9v battery for a particular load.