So when looking at new star systems how can you tell the benefits of each planet to see if it's worth colonizing? For instance, one planet might be 2 bread, 3 hammer and 1 gold. Whereas, another planet could be 1b, 2h, 6g.
Technically every system is worth colonizing, the real question is more a 'when' than an 'if'.
I think you should settle all systems surrounding the Forge, fill them with buildings, and leave only token garrison in place.
If we ignore geopolitical concerns (like needing to cut another faction off by settling a system near a chokepoint or something), there are four factors determining the quality of a system. In no particular order:
#1: Number of Planets
Duh, the more the better
#2: Size of Planets
Duh, the bigger the better
#3: Good Planets in the first and second 'cultural' ring(s)
This is less important for Paradise and more important for Halis. I've had systems where the only immediately accessible planets were two size 1 dwarfs, with nothing in the second ring but three size 3 planets with a resource and moons in the third ring. Such a system might become very good in the long run, but it takes ages of spamming influence buildings just to get to work the good planets. Ideally you want the best (allow for big populations, have a resource and moon(s)) planets in the first or maybe second ring.
#4: Specials
Mostly resources, but moons are also good to have especially in foodrich systems. Rings might be worth considering if you have your eye on that one world (universe?) wonder that requires them. New Earth and Paradise should prioritize systems with a (happiness) resource in their first ring to counteract their trait given penalty.
I did say that those factors were in no particular order, however I have to add that one size 3 planet is usually better than 3 size 1 planets. Quality beats quantity here.
Also, how do you choose what planet on which to build the item in your production queue? I don't mean how is it done, but what should be factored in the decision? Or does it not really matter anyway?
Depends. Most buildings you can just place anywhere, however you need to pay more attention to buildings that only boost the planet they are on like Mining Facilities, Mag-Lev Networks and Habitation Systems. Those should be built on your best and biggest planets to get the most out of them. Also notice that some buildings have upgraded versions that can only be built on planets that have the basic version already, e.g. Factory and Manufacturing Plant or Mining Facility and Nanoextraction Upgrade. Since buildings become more expensive the more copies of them you built in the same star system it is better to concentrate the planet specific ones rather than spread them out. This is also the reason why one big planet is better than several small ones. More than two instances of the same building in one star system are only rarely profitable, although I will sometimes make a third instance of a building if its production is boosted by my trait.
Remind me to take screenshots once my next turn comes around to show you specific examples.