A doctored, phony image of President Barack Obama shaking hands with President Hassan Rouhani of Iran. A real photograph of a Muslim girl at a desk doing her homework with Donald J. Trump looming in the background on television.

It is not always easy to tell the difference between real and fake photographs. But the pressure to get it right has never been more urgent as the amount of false political content online continues to rise.

On Tuesday, Jigsaw, a company that develops cutting-edge tech and is owned by Google’s parent, unveiled a free tool that researchers said could help journalists spot doctored photographs — even ones created with the help of artificial intelligence.

Jigsaw, known as Google Ideas when it was founded, said it was testing the tool, called Assembler, with more than a dozen news and fact-checking organizations around the world. They include Animal Politico in Mexico, Rappler in the Philippines and Agence France-Presse. It does not plan to offer the tool to the public.