No, it's a good question. What makes it a good question is that it is hard to answer. The best guide is the rating of the ship. If they are marked dangerous they probably have good gear. Harmless or mostly harmless are likely to be easier. You should also, as you've noted, keep in mind the ship types. It's unlikely that you'll take out an anaconda or python in a sidewinder, but you might be able to take a cobra or eagle, all depending upon skill and loadout.
I would recommend starting out by just testing yourself in situations where you can fight 1 on 1. Start small, other sidewinders, then haulers, then eagles, then Vipers, then Cobras, then Asps. By the time you're comfortable with those you'll likely be in a better position to judge the rest.
I would also suggest investing in a shield booster as it will let you reinforce your shields if you get into a scrap and are out classed (or just want to dump everything into engines and weapons).
Best of luck!