In my experience, half of the time, I can look at a deck and say to myself, “this is totally cohesive, powerful, and dynamic,” or, “this deck is pretty good, but has a few kinks to be worked out. It just needs some tuning or one more card to come out and it will be good”. Some good examples were cambridge PE, Prepaid Kate, Eater Maxx, and HB Rush. Usually, you can get a good idea about these things from knowing about previously built, similar decks, and recognizing that an improvement has been made, In the case of Prepaid, you could lose ProCons, which was always a high variance card that led to a lot of shitty starts. In the case of Cambridge, mushin solved an econ issue while also putting a serious threat to the runner. In the case of Eater Maxx, you solved Siphon anarch inconsistency issues by only needing one breaker, and in the case of HB Rush, you just added a bunch of powerful cards, DBS, Eve, and Ash, into an already successful HBFA deck.
In other cases, a deck might look a little sketchy on paper, but once you see a couple of games, you realize it works. NEARPAD and @SamRS’s Reina Headlock were examples of this for me. You see them matched up against good players and good decks and win, and that’s pretty much all you need to know.
In my experience, pretty much every deck I come across needs some amount of tuning. Usually what I do is take the concept of a deck, tune it to my liking in a deck builder, look at all the cards available in faction or splashable with flex influence to make sure it’s the best it can be, and take another look. If there isn’t another deck that does the same thing better, and the tuned list looks like it could some relevant good matchups, then I play a few games. If you’re a good enough to player to build decks, you should be able to do most of the tuning without actually playing the deck at all. If you’re not at that point, you should play more with other people’s decks.
It really doesn’t take too many games to make the final adjustments on a deck. It also doesn’t take too many games to figure out if it’s good. If you can find a strong opponent with a strong deck and win half the time, the deck is worth it. If you’re losing a lot, ditch it. Most decks can’t be saved from having multiple 25% matchups against top tier decks by tuning, so if you’re losing that much right off the bat, I would let go. NBN tagstorm (both Psycho-Beale and Scorch) is a deck that has had success in the past, so there is no reason to think that it isn’t powerful enough to win a decent amount now. You’re doing yourself a disservice by not copying someone else’s successful deck to start; there’s no reason to start from scratch when the scaffolding is already up.