There are two primary dangers in low sec: gate camps and roaming pirates. if you're just passing through, then you will simply be warping gate to gate so you only have to worry about the former.
Avoiding Gate Camps:
By far the most heavily camped gates in low sec are the ones that border either high sec (0.5+) or null sec (0.0). Presumably, you won't be passing through null sec just yet, so the main system you need to worry about being killed in is the one you enter when you first leave high sec (more on why you don't need to worry so much about the low/hi border at the other end of the trip later).
The star map is your friend here. Press F10 to bring up the map. If you're in solar system mode, click the "toggle map" button on the map control panel to switch to galactic star map. If you've already set a destination, then your route should be highlighted, and the names and security status of the systems you'll be flying through should appear down the left hand side. Look for the first system en route that has a security status less than 0.5 and locate it on the map (you may find the "flatten map" button on the map control panel useful or not, depending on how good your 3d visualisation is). Now, in the control panel, click on the "star map" tab and then the "color stars by" subtab. In the "statistics" folder, select "Ships destroyed in the last hour". Obviously, if the system you're looking at flying into lights up like a carnival, you might want to reconsider your travel plans.
If you want a little more information, also check the "average pilots in space in the last 30 minutes" option. If there are no or few ships destroyed, but more than about ten people in space, you might want to consider the possibility that a gate camp is in the process of assembling. Likewise, if there were a ton of ships destroyed, but there are now only a few people in space, it's possible that a gatecamp has recently disbanded. Either that, or that a large fleet battle recently transpired and everyone is now licking their wounds (which can make for some really good ninja looting if you're bold).
Most of space is empty, but don't get too spooked just because there are a few people in the system you're passing through. A lot of them will just be minding their own business (or will be roaming pirates without the logistics to set up a gate camp). It takes generally at least three people to set up a decent gatecamp (one heavily tanked battleship to get aggro from the gate guns, one logistics ship to keep that tank ship alive, and at least one DPS ship). In practice, most gatecamps will have at least 7 or 8 people, because anything too minimal is just begging to be busted by an anti-pirate pvp fleet (a single logistics ship can be destroyed rather easily if you're well prepared), and after that the gate guns will help chew through the rest of the camp very quickly), so if there are fewer people than that in space you can feel relatively secure passing through.
If you decide to bypass a specific system, right click on the star in the map and choose "Avoid solar system". A new route will automatically be plotted for you. Keep in mind that the avoided system will stay on your nav computer avoid list until you remove it in the "avoided systems" tab, so you'll want to clean that out occasionally.
Still, despite your best efforts, you'll sometimes find yourself greeted by a dozen or so bright red bars on your overview when first entering the system. All is not lost.