What do you mean by 'class'? The
size of a ship is one of the obvious things that you should always be able to see.
But, the
mission of the ship, attack ship, mine sweeper, carrier, etc... now that should ne unknown until you get close enough to scan it or you actually meet it in combat.
I think the most important thing of all is that 'vision' in a system not be absolute as it has been in all SE Versions so far. There should be definite range degredation of normal sensors so you cannot see a ship of a given size beyond a certain range unless it is broadcasting some sort of 'ID' signal. And stealth or cloaking would reduce this range even more, of course. And rather than have things suddenly 'appear' clear as day there ought to be a gradual increase of available information.
First you sould get the simple notice of a ship of a given size at whatever range it first becomes detectable. This could be represented as a generic 'ship' outline with question marks or something. Then some details of things like configuration (meaning external design factors in this case -- the 'silhouette' of the ship). After a while you could learn to ID many ships simply by their silhouette of course.
Then finally some technical details like the type of engines it uses or whether it has shields (and how strong those shields are) could become detectable. It makes sense for engines and shields and certain other technologies to be identifiable at long range. They produce a lot of emissions. Only in actual combat (or with special scanners) should you be able to see all the components, with an important exception. BUT... that brings up long range scanners.
Long range scanners should not tell you about components that YOU don't understand the technology for! How would you be able to recognize organic armor if you don't know what it is? There should be a 'blank' for these unknown components. Maybe each component should have an extra description, the 'unknown component' field which would be shown to enemies who do not or cannot possess the given technology. And also, the scanner jammer should work in combat like that do at long range and completely block scanning so you have no idea at all what the other ship contains.