Keeping up on the communities will help you know what decks are going to be hot; knowing the players, who is popular and relevant. Basically just being a part of the community, paying attention and watching how things sell will inform you. There aren't any tricks or shortcuts. You can reliably assume Lotrek, Fontaine, JR etc will always be somewhat of a Veblen Good, and a reliable buy.
What you are going to continue to hear is "buy what you like", and while I agree with this to an extent, it's important to factor in why you collect. And I can offer a different perspective: I am someone who gains an immense amount of pleasure from owning things other people don't. I buy what I know people will be jealous of. I have a pool of money set aside for new releases I know will fit that concept. The decks I cherish most aren't my favorite decks, they are the decks I know other people wish they had. And I like to feel like I am part of a private club of elite collectors that own the rare stuff and know that having a Tatler is more impressive than having a red Fontaine. I like to have decks that other long time established collectors of modern decks appreciate but the kiddies on Reddit have never even heard of. And I like to flex my collection.
That said, I still always get decks that resonate with me, and buy a couple extra when one really sings to me. I had to step away from collecting for several years when collecting became unhealthy, I was chasing trends, trying to play to valuation game and spending more money than I had on decks I didn't even like, just to be a completionist, or because everyone else was going to have one of whatever deck and I couldn't handle not being a part of that. I stopped collecting because I loved cards, and ultimately that should steer your decisions to buy. Coming back to it now, I am finding it much easier to say no to decks, it has to really be something that speaks to me. I'm still putting money towards that envy pool, because decks that other people want definitely still speak to me, but I finally am only getting decks I feel a worth having, and I set money aside to make sure I don't miss a release that I would kick myself for missing.