Don't put an ohmmeter across the terminals of a battery! You could destroy it.
You want to test voltage under load, which is different. A load (given in ohms) is placed across the battery, and the voltage is measured. If the battery isn't up to the task, the voltage will drop below a specified threshhold.
Some people use the name ohm meter when they mean multimeter. Just make sure your meter is set to DC and the range is set so the reading you want will be as close to midscale as possiable. A good rule is to start at a higher range and work down. This will make sure you dont peg the meter and harm it.
There are 3 tests that you can do using a Digital Multimeter (DM) (This is a meter that can be used as a volt meter, a current meter and a resistance meter.) to determine the suitability of an alkaline 9 volt battery for rocket electronics. This applies to 9 volt alkaline batteries only.
First. A word of caution. Never connect a multimeter to a battery if the ohms scaleis selected. It will at minimum blow the input fuse, or if your multimeter doesn't have one, it can destroy the meter.
Test 1. Is it alive? Do a no load test to find out.
Set the DM to DC volts and if it's not an autoranging meter, set the scale to 10 volts. Touch the probes across to the battery terminals. (Caution: Do not short the two terminals with one probe.)
The meter should read around 9 volts. With any thing less than 8.5 volts, don't bother going to Test 2 since the battery isn't very good and should not be used in a critical application.
Test 2. How much life is left in the battery? Do a load test to find out.
Go to Radio Shack or any other electronics store and get a 1000 ohm resistor (1/8 watt is fine, 0.25 or 0.5 watt is better.) and a 9 volt battery clip. Solder or twist the wires from the batery clip to the resistor. Snap the test rig to the battey and measure the voltage across the resistor. It should be between 7 and 9 volts.
At 7 volts your battery is on it's last legs. The % lifetime left in the battery is approximately described by the equation
% Lifetime Left = (V measured - 7.0)/7.0
If the battery is below 8.5 volts here, there's plenty of life left in it, but just not in your rocket.
Test 3. Is it really good for my rocket? The shorting test. Only required if the battery is going to fire pyrotechnics. For altitude recording only, you can ignore this test if you passed test 2.
Two words of caution.
First, your meter must have a 10 amp scale. If it doesn't you can do this test.
Second. Do this test for only 1 or 2 seconds. The life-time of a shorted 9 volt battery is only about 72 seconds!
Put the DM on the 10 amp DC scale setting. Make sure the hot probe are plugged in the 10 amp socket as well if you DM has more than one amp position. Hold one probe to the battery and quickly touch the second probe to the second terminal. The meter should read at least 2 amps. If you don't get 2 amps, use the battery for something else. It's a good battery, just not good enough for your rocket.
A really fresh 9 volt alkaline battery can put out 5 amps for a short time period into a short, but since it only take a few milleseconds to fire a typical e-match at 2 amps, you don't need much time at the high current.