I mean if I check using copyscape when buying articles is it going to be fool-proof way of knowing if it was never published on internet? I understand it would not catch those articles who haven't been indexed yet by say Google (within 6 weeks or so).
Copyscape definately is one of the leaders out there although in additon you could also Google select quotes from articles of interest to see how many results come up.
For example you can copy paste something like "I highly recommend resturaunt x in NYC for high quality goods" (with the quotes) or some fairly unique phrase that you wouldn't think of randomly and gauge the results to get an additional idea.
Having a reliable source for content also is crucial. Sites like Associated Content are typically spammy articles and since they're free, it's copied quite often. Using a copywriter or going to a niche area however should provide better copy.