How can I check if my bike is safe for touring? I have an old second-hand bike that I use daily for commute, and in a few weeks I want to take it on my first bike tour around Europe. 

I'm a bit worried, since the bike is quite old (has some rust spots, a loose spoke or 2, etc.), that it might not be the safest thing to do. However it never had any problems while riding around the city, and unfortunately I don't really have money for a new bike or parts.

Are there some standard checks one can do to see if it's fit for touring? Are there any serious problems that I should be worried about?
Get the bike serviced, giving yourself enough time to thoroughly test it, so that if the service introduces any problems, you discover them long before your trip.

There is no standard test for worthiness, you'll have to rely on your judgement.