If the 650 was like the 1000, then it would be a very simple valve tappet check with a feeler gauge.
It is not though, it uses shims to adjust the valve clearances. Just for some personal feedback, I had my 06 model checked at the shop at 18,000km as per the book, they were in spec.
My 07 model was never checked, I clocked up 90,000km while riding the Americas
My 09 current bike, has just clocked 56,000km and never been checked and I am currently in Europe riding it home to Oz
It is NOT something I would do unless I had a very good clean garage and all the right tools, but as I am on the road, that is not possible and I don't trust anyone else in there
If I were you and worried about it, I would pay the few dollars(maybe 1.5hrs labour) to get the Suzuki shop to check it before going on a long trip
But, from the information I have from VSRI.com, you can get away without it, but it is your decision to not do it or do it as you feel fit
At this stage of the bikes life, it will need a new air filter, and fit some bug screen over the air intake snorkle to stop them getting inside. Make sure you ask for an 07 airfilter WITH gasket, do not throw the old one away in case you need to re-use the gasket, my 06 bike was dusted because the mechanic left the gasket off at the 18,000 service
New spark plugs
It will probably need a coolant change and bleed the brakes with new fluid because it is over two years old, just check the manual for the required replacement times
There is not much else it really needs, just change the oil and filter everytime at around the 5-6,000mile, or 8,000km and it will last for ever
Cheers from Austria