I am planning to visit Madrid and have been dealing with a firm "Offshore international trust". They have a website as well www.internationaloffshoretrust.com and is located at Paseo de Castellana 330, 28004, Madrid. How do I find out if it is an authentic firm or not. I am trying to protect against some online fraud.

Castellana 330? that address does not exist, Castellana ends up in #300
And Castellana 300 something is not 28004, that post code is central Madrid and the (supposed) address is outside the city.
This address is almost surely false or misread.

UPDATE: I've checked the website you provided. The address DOES NOT EXIST, the phones are CELL PHONES. The company does not show in the yellow pages. ALERT. It's a fraud.

Moreover, the design of the Webpage is a plagiarism of the old version of the Website of a reliable and well-known Spanish bank, Bankinter

It's frightening, it is really a clumsy copy of the BK site with a clear intention to deceive. FRAUD