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Recently my old amplifier in car has got a defective contact on one of the switches, so I decided to unsolder the old one and solder a new one. Unfortunately the amplifier caused a short as I connected it to the battery again (I don't know how this could happen, maybe I damaged some component or something, or maybe there was suddendly a wrong connection due to soldering althought I don't think so).
What I've done next is to unsolder the switcher again and bypassed the needed conducting paths by soldering some kind of bridge (I don't need to switch it again, it's ok for me when it just works with a specific adjustment). But before I try to connect it to the battery again, I'd like to test if this is save. So I thought about using an ohmmeter. My idea is when I connect it to the plus pin and minus pin of the amplifier, the resistance must not be 0 Ohm, because this would cause a short since 12V / 0 Ohm means infinite current. Are my thoughts correct? Or do I miss something important?

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The Ohmmeter will do fine, you are correct in thinking there should not be 0 Ohms (read very low resistance e.g. <10 Ohms or so) between the amplifier power leads.
A simple continuity tester would do for testing for a short, but the Ohmmeter actually gives more information as you can see the actual resistance value.
You should probably read a few hundred ohms if things are normal, try it out and let us know what you find out.

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Thanks, it showed about 900 Ohm now and it doesn't cause a short anymore. Obviously my bypass worked this time. – Bevor Jul 28 '12 at 8:28
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While I am not sure exactly what you have done (pictures or a schematic would be helpful) I do know that you can measure if two wires are connected by using the method you described. However, the best way to check this is with a continuity checker. If the ohmmeter you bought can also measure other things, I would read the instructions to see if it can also measure continuity. If you live near a harbor freight, they sell multimeters with this option for about 5 bucks. They also sell continuity checkers for a little less. If you plan on doing electronics much in the future I would definitely invest in a multimeter with a continuity checker. They are nice because you just place the probes on the two nodes you are interested in and it will audibly beep if the two nodes are connected. Hope this helps!

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My multimeter can do continuity check too. But honestly, I was unsure about the result of this test due to my shallow knowledge in measurement. – Bevor Jul 28 '12 at 8:33
The continuity measure is great. The only time I have found that it measures incorrectly is across an inductor. An inductor reads a short because that's what an inductor looks like in 'DC land' Beyond that, if it beeps cleanly, you are good to go. Make push the sharp part of your probes into the solder joins a little (enough to make a small dent) If it crackles, there is a connection there, but it may be a poor connection. What exactly are you unsure of when measuring? – Danny Kmack Jul 28 '12 at 8:44
I wasn't sure if a beep would be good or not, ok I could have been deduced it from the fact that connected test prods to one other make a beep, but I'd rather don't make experiments with electric things I'm not sure about. – Bevor Jul 28 '12 at 12:05

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