As far as we know, the Nazis were unaware of the pending Pearl Harbor attack and the Japanese weren’t sure if the Nazis would declare war on the USA.

The Nazis, specifically Joachim Von Ribbentrop (Germany's Foreign Minister) had been informed by Ōshima Hiroshi (Japan's Ambassador to Germany) in December 1941 that relations between the USA and Japan were at a breaking point (because of the USA's trade embargo on Japan) and that war was imminent but did not mention any specific plans to attack Pearl Harbor.

Under the Nazi/Japanese alliance agreement (Tripartite Pact) in place, Germany was only obliged to come to Japan's aid if a third country attacked Japan, but not if Japan attacked a third country (i.e. the USA). So as far as Japan is concerned, the Nazis were under no obligation to join them after the Pearl Harbor attack.

After the Pearl Harbor attack, Ribbentrop specifically bought this to Hitler's attention, adding that to declare war against the USA would add the number of enemies Germany was fighting but Hitler essentially waved him away, saying “this concern is not important, and without any consultation with his Generals (or anyone else) chose to declare war on the USA.

I say spur on the moment, usually it’s argued that Hitler agreed to invade the USA because:

1. He was inspired by Japan’s surprise attack on the USA (being a fan of surprise attacks himself)
2. He he thought the USA would declare war on Germany soon anyway (based on the USA’s push towards a wartime economy)
3. He saw the USA as a country “heavily under the influence of Jews and Negroes” and that the USA needed authoritarian discipline to control them.
4. He believed that Japan would then declare war on the Soviet Union, easing Nazi resources and men on the Eastern front, and also that Japan would start attacking the British colonies in Asia, which would make the British withdraw most of its sizeable forces from Northern Africa to the Pacific theatre. In the end only the second part was correct.

So if you look at it objectively, the Nazis didn’t know about the Pearl Harbor attack and had made no official promises to Japan to invade the USA. Germany only joined the war on the USA because Hitler, on the spur of the moment decided to join the war. Whether the Japanese were confident that Hitler would declare war based on the 4 above points is something that we simply cannot know.

If you want to go further, in March 1941 the US economy had already started gearing for war (vastly improving its armed forces, aircrafts, ships etc) and was well on its way to a complete wartime economy by December 1941. So it’s understandable why the Japanese would have expected Hitler to believe that the US would have enter the war soon (on the side of the allies) and want to attack the USA first, with Japan.

Some argue that before the Pearl Harbor attack, Ribbentrop and Hiroshi had drafted an amendment to the pact, which stated that Germany, Japan, and Italy would pledge each other allegiance in case one is attacked or attacks the USA. This amendment hadn’t officially been signed, but the argument is that it’s mere existence, which would have been discussed in depth by high powered officials, would suggest that Nazi Germany were ready to go to war with the USA and thus Japan would have been confident of a German declaration of war on the USA.