It’s really not that hard.
C6 = deadly
C1 = medium ships only
So that leaves 2-5 and small entrances.
Yes you can look at the color of the hole and that can work. But if you don’t want to do that, here’s a different way to do it.
Look at the combat sites. Generally, they are class specific so if you know the naming convention, you’d know the hole class. There’s an exception for shattered WHs but there are not too many of those.
In addition, if the WH has an effect, the effect has a level and that level usually corresponds to the hole class (there are also a few exceptions to this).
That said, you can nail probably 90% of holes without visual inspection of the hole and without a third party site.