There are a few things you can do to help understand runner decks.

One of the first things I'd do is look at what kind of economy they are running. A drip economy deck like data folding and underworld contacts want to spend time setting up so it will be unstoppable later. A day job deck wants to apply a lot of pressure very quickly, pause to recover, and do it again. An armitage/liberated accounts/katiJ deck wants to apply a little steady pressure each turn, always keeping the threat of running available with backlogged available credits. A prepaid deck wants to ride credit busts like a rollercoaster, getting a bust of credits, making a few runs, slowing down a tad then bursting Forward again. A recurring credits deck wants to make one to maybe two good runs a turn.

The next spot I'd look is their multi access. Event based multi access means they want to apply pressure elsewhere, and then hit that area when it's weak. Hardware based multi access means they want to apply one good run every turn or two on a regular basis. Virus based multi access means then want to spend a turn just hammering something and force you to deal with it.

So a deck with medium and legwork wants to hammer r&d and keep up with remotes, and then once this pressure forces you to spend time locking up r&d and keeping agendas in your hand, legwork in. A deck with hq interface and makes eye want to see hq fairly often, forcing you to put agendas in servers and defend them, and then will make a run for the remote, and when you spend everything defending that, they'll makers eye r&D for that big pull .

After this look at breakers. How quick do they set up, how efficient are they? A deck with torch and garrotte is going to set up slow and be unstoppable late. A deck with inti and zu is going to be able to set up and start stealing turn two, but the longer the game goes it gets tough to keep up. A deck with eater is going to do ice destruction and access replacement affects ( account siphon and keyhole). A deck with cassia is going to pick a server and hammer it.

Also look at card draw. Is it a tutor based deck on special orders or smc/clone chip? Is it event based draw such as diesel/I've had worse? Big draw like game day and quality time? Repeated draw like wyldside or earthrise? Draw improvement effects like Mr li/professional contacts/symmetrical visage? They can tell you if a deck wants to find the right card like a silver bullet, or just constantly play a card or two a turn, or spend everything then reload.

Now the quickest way of all to get right to the core of a deck - look at the console. The console generally fits the theme of the deck. Doppleganger decks want to run a lot in one turn. Desperado decks want to keep constantly gaining cash and out money corps. Deep red decks want to cassia, spinal modem decks want to make efficient runs, comet decks want to chain events, etc. Astrolabe decks want to play efficient things and keep drawing into them, logos decks want to get the right pieces and set up. Usually the console is pretty in line with the deck theme.

Finally look for extras. Mu is usually pretty tight, so programs that don't break ice are important to notice. Sneakdoor beta betrays certain strageies. Crescentus and parasite are some pretty impactful effects. D4v1d and imp can tell you a lot as well. Lamprey means they want to hammer the corps econ and force them to waste turns. Non programs are important too. Blackmail means they want to run centrails and never let ice rez on remotes. Xanadu forces that taxing aspect. Vamp and account siphon make the Corp poor for a turn. Emergency shutdown/forged activation force the Corp to lose money rerezing or destroy ice, or maybe just spend money in a different area this turn, leaving you free to run the area you want. Parasite/cutlery will make runs efficient by destroying taxing ice.

Every card naturally wants to be economy or card draw or breakers or muliaccess for the normal "find breakers and play them, then spend money accessing things.". So if something isn't one of those four, it usually has a very very good reason for being in the deck.

Take a few online decks, look at these areas, and see if you can write down where they fall in each one, and if you can figure out how they run from all those characteristics.