I have a problem. My N900 usb socket was broken so I gave it to local service. After rapair I noticed that my mainboard might have been replaced during that service. I have scratches on camera lens and in some hidden place. These scratches "magically" disappeared. IMEI number is correct and data on eMMC are the same. I had lock code - it is also correct. But I don't know where is it stored. So there is a question - is there any chance to replace or copy eMMC memory and lock code; to change IMEI and so on?
And generally - how to check if my mainboard wasn't replaced?

It is very unlikely that any spare motherboards for N900 would exist at all, so I'd try to find an alternate explanation...

I have heard that there are many N900's which have problems with mainboard (usb rapaired many times, overheated etc.). So I wonder if this "private" serviceman did not replaced my motherboard with any of described above.

Is there a possibility of changing IMEI, BT adres and WLAN address permanently? And if so how can I check if motherboard installed in my N900 is really original one?

That's highly unlikely, you can't change IMEI AFAIK.
And MAC:adress can't be change permanently, AFAIK. It can be spoofed but that resets after reboot unless there's a script at boot time but that will be reset when reflashed.

Anyhow I find that you are way too suspicious, for multiple reasons it's highly unlikely that he replaced your motherboard.

1) IMEI changing is not trivial.
2) Changing MAC permanently, could be done but not flashing-survivable.
3) Not that many N900 spare parts left in stores/servicecenters, if any. Let alone a whole PCB. If there's any it would be more assumable that is a cameramodule. (And he replaced the camera. )
4) What does this repairman gain by replacing the PCB? I don't see any advantages for him.
5) Soldering/desoldering eMMC not feasible without some very highend machines.
6) Changing lock-code can't be done without reflashing, so if he wanted to clone you're device he had to gain access to your device first which can't when you have a lockcode. Unless you had bootmenu, recovery-console/backupmenu installed, this won't happen without reflash.