Mar’s apparent retrograde would be real obvious if Jupiter or Saturn were nearby enough to easily compare their angles with reasonable precision or even eyeball precision. And once noticed, folks would be on the lookout for it to recur. Which it does roughly every 2 Earth years. Experimental confirmation would not be too long in coming.
Yes, Jupiter & Saturn apparently retrograde too, and are always moving even when not retrograding. But IMO for the smallish number of days Mars is in retrograde (wiki sez 72), the gas giants won’t have moved so much as to be useless as references.
In a world where the only visible celestial bodies are the moon, the sun, and the five planets that can be seen by the naked eye, the sky would be much darker at night, especially when the moon is new. In our world, people looking up at night were able to discern all five visible planets and note that they were different from everything else in the sky. In this world, these would be the ONLY lights in the sky (aside from debris occasionally burning up in the atmosphere; assuming that still happens at a similar rate to our world, shooting stars should be much easier to see). So people would be paying them lots of attention, very early on. I think navigation methods would eventually develop based on the planets, even if it would require much more complex math (and may, in fact, spur the developmeny of some types of mathematics earlier than in our world).