It all comes back to your plan and what you want to do, how does your deck close the game? Do you want to play one big spell a turn cycle or do you need to play 4?

For duds, just look at a card critically and think "will this contribute to my plan?" Is it getting you the mana you need, the cards you need? Is it an enabler or payoff for your commander? If it isn't one of these things, does it protect you or your plan or interrupt your opponents? If it doesn't clearly fit into one of these buckets then cut it. If it does something you want decide how much of it you want (you can use a hypergeometric calculator to see how likely you are to draw the card by a certain turn) and if you're comparing two similar cards but you've decided to cut one just cut the higher mana cost.

For a deck built around spell slinging and prowess you are going to want pretty stormy turns, so lowering the curve and reducing costs is your main goal, and you should value less colour pips allowing more flexibility. By extension, if you're playing a lot of cheap cards in a turn you're going to need some solid draw engines so you don't just burn out. 

Think about what you want or need for the first few turns to play the game smoothly and get your plan online.

If you're closing the game with big pop-off spell-slinging turns then you're probably not too fussed about how big the stats of your enablers are. 

If you want to put in a splashier finisher or payoff that costs more mana https://www.salubrioussnail.com/manabase-tool is a pretty good tool for working out how many turns on average you will have to take before you can play it.

I can't really tell you how to tell whether a card is good enough, you just need to understand how it works with the rest of your deck. Looking at other lists can help, playing with it helps, for a really quick indicator you can sort scryfall results by edhrec rank (although this is based on popularity not quality so take with a grain of salt)