How do I find the signs that planets are in during any given date? Say for example I wanted to know where the planets were on December 19th, 1664, at 1am; how do I figure that out?
You can use the software Stellarium, which is available on most plateforms, and in-browser. With this software, you can show the constellations and the planets at any time and date, and see where the planets are in the sky.

For instance, here you can see that Mars is in Libra and Mercury is in Scorpius. 


At the dates you specified, the positions of the planets were the following