Today a friend told me that my exhaust gas smelled funny and that I should check my valves. I don't find it very likely since the engine is around 6000kms old. Can I avoid the trouble by checking the compression? If the valves need adjustment the compression should be low right? Even if good compression not necessarily mean good valve adjustment, if I get a good compression, should I still tear down the engine head?
No. A compression test is used to measure the sealing in the cylinder, both through the valves and the rings. You cannot confer anything through a compression test as to the adjustment of the valves, because it doesn't relate to that. You may get compression readings which are less than they should be if the adjustment is too tight (a tight adjustment might keep the valve open more than it should be), but will tell you nothing if it is too loose. It also depends on which valve the adjustment is loose/tight on. Just because compression readings may be low does not automatically mean your valves need adjustment. All a low compression reading is telling you is that something isn't sealing correctly. It could be due to a bad valve to seat interface. Like I said, you cannot use the compression test to directly tell you what you this information.
You shouldn't need to tear down the head to get a valve adjustment. Depending on the engine, you can usually get to and measure the valve lash by taking the valve cover off and measuring directly at the valve tip, or between the cam and the follower.
If compression is good then you don't need to tear down the heads. Compression might or might not tell you if the valves are out of adjustment. This has to do with valve overlap and timing.
Tight valves open up sooner, open up wider, and stay open longer, which too tight can lower compression and burn exhaust valves; compression loss due to valve not seating quick enough on the compression stroke.
Loose valves open up later, open up narrower, and close sooner, which can cause back pressure and loss in power/mpg from excessive vacuum being pulled on the intake and power stroke. (intake valves don't open when supposed to, creating excessive vacuum during intake stroke; exhaust valves don't open before end of power stroke creating excessive vacuum and also close to early at end of exhaust stroke creating unneeded compression.)