How do I determine the value of a signed Salvador Dali lithograph?

A coworker of mine has a signed lithograph of a Salvador Dali painting - ';Slave Market with Disappearing Bust of Voltaire';





The lithograph is numbered 142/300 and the signature appears to be genuine.





Assuming it's genuine (i know that's not guaranteed), how would we go about determining its value?How do I determine the value of a signed Salvador Dali lithograph?
Check for something similar to it on the web. Bbay has loads of Dalis for sale. Also, where did he get it? If he has the bill of sale from the gallery he bought it from, and they are still in business - call them up. Do eveything you can to establish provenance...as there are LOTS of fake signed Dali works out there.

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