Telling which class of w-space system you are about to enter appears to be an art, or perhaps some kind of witchcraft. To the untrained capsuleer the indecipherable letter and numbers that float near the wormhole mean little, and coming at it from the wrong side only gives you the standard K162 designation that gives no information about the system beyond. It is possible to interrogate a database of static wormhole designations in order to determine the class of w-space you're about to enter, but it would be much more useful to be able to tell at a glance. Trying to inform the inexperienced pilot about the colours that bleed through somehow only leads to more confusion. As a parting gesture, let's see if I can make visual identification easier.
First, wormholes are not simply holes in space, they are more akin to tunnels. The colours that surround a wormhole reflect not only the system on the other side but also that of the system you're currently in. The outer edge of the wormhole represents local space being funnelled through the portal before it transitions entirely in to the view of the system beyond. This is important to realise, as the different colours around the edge of the wormhole can potentially taint your impression of the whole.
Despite wormhole colours varying from edge to centre, the wormholes themselves appear to be in some kind of cosmic alignment, as any reflection of the current system's nebula shimmering around the edge of the wormhole doesn't seem to interfere with any view of the nebula through any wormhole. Never the less, along with example images of the main nebula colours as seen through a wormhole, I also include a more comprehensive matrix to aid with identification and visualisation.
This guide only provides information about w-space systems. I have written a separate guide for New Eden wormholes that lead to high-, low-, or null-sec space.
Along with the visual cues, the information panel provides most of the information about a wormhole, including the maximum individual ship mass that can transit, the health of the wormhole, and the tier of w-space the wormhole leads to. There are three tiers of w-space, with class 1 to 3 systems being 'unknown' w-space, class 4 and 5 systems 'dangerous' w-space, and class 6 systems 'deadly' w-space. Identifying class 6 w-space systems is therefore pretty straightforward. It is also perhaps the easiest to identify visually, having a vivid red colouring.
Moving down to merely 'dangerous' systems, the class 5 wormhole can be recognised by its mix of orange and pink hues.
Wormholes leading to also-dangerous class 4 w-space are distinctive from the lower-class w-space systems by the tinge of red to the purple-grey nebula.
Wormholes to the classes of 'unknown' space are fairly similar, but there are still distinctive patterns to tell each class apart visually. Class 3 w-space systems have a purple splotch amongst the predominantly grey colouring.
There is a distinctive black gap in the view to a class 2 w-space system, looking like a bear trying to catch a leaping salmon.
Wormholes to class 1 w-space systems are perhaps the least distinctive of all in their colouring, being a rather uniform grey-purple.
Thankfully, theses wormholes have the excellent benefit of being unique in their individual ship transit mass allowance. Look for the teal aurora surrounding a wormhole to be assured that you're travelling to or from a class 1 w-space system. This illustrates the extra information available from the auroras surrounding the wormhole, not just the nebula colours. The auroras that swirl and dance, wax and wane, indicate the maximum individual ship mass that can transit the wormhole.
A royal blue aurora indicates wormholes that only allow frigate-mass ships. This is the most important one to recognise, particularly when approaching the K162 side.
A wormhole with a teal aurora won't allow battleships through, and is the indication of a wormhole connecting to or from class 1 w-space.
A turquoise aurora highlights a wormhole that allows every ship below capital hulls.
A yellow aurora indicates wormholes that freighters can transit.
Note that the auroras indicate the individual ship mass limits for the wormhole and don't change with the total accumulated mass passed through the wormhole.
Now let's look at how the originating system affects the colour of the wormhole. I don't think this is a particularly necessary step, because of the cosmic alignment of wormholes. But as your ship will rarely approach a wormhole from exactly the right direction, I believe there is still use in seeing the nebulae slightly askew, as well as having the current system's colours bouncing off the edges.
I have collected a range of images from many types of wormholes to illustrate the differences. The matrix format should make both the originating and destination system colours clear. The rows show the originating system and the appropriate colours are reflected around the edge of the wormhole. The columns show the destination system and the appropriate colours are reflected in the centre of the wormhole. There are currently gaps in the matrix, although I aim to update the table as I encounter the missing types. As with the above images, a larger version of the image lurks behind each wormhole.
And for those who prefer a black background:
[Not forgetting the corresponding direct link to the table.]
Hopefully with this information you will be able to tell quickly and easily the class of w-space you are about to enter. Being able to do so at a glance removes the need to jump in to the system to find out, particularly if you are looking at the K162 side of the wormhole.