A group of senators filed a petition against Mr Samak, saying that a prime minister is barred by the constitution from working for private companies.
Mr Samak told the Constitutional Court on Monday that he had been paid for his appearances on the TV show, but was not actually employed by the television company.
The court is due to give its verdict on Tuesday.
Local newspapers have speculated about the case being a possible short-term solution to the current political stalemate in Thailand - although some analysts point out that even if Mr Samak stepped down, his party could always vote him back in again.
For the past two weeks, protesters have been camping outside Government House, refusing to leave until Mr Samak quits, but despite pressure from several military leaders the prime minister has refused to go.
Late last week, Mr Samak proposed to hold a referendum on his rule, but the opposition has rejected the offer.
The demonstrators, from the People's Alliance for Democracy (PAD), say Mr Samak is merely a proxy for former PM Thaksin Shinawatra, who was ousted in an army coup in 2006.
One man was killed in clashes between pro- and anti-government groups in Bangkok last week, prompting the government to impose emergency rule in the capital.
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