I think it's hard to tell from the appearance, texture, or aroma of the dry leaf. I wouldn't want to even venture a guess about the processing parameters of an oolong without at least kind of brewing the leaves one way or another.
Things I would look at, though, especially if you can brew the tea:
1) Low to medium oxidation will give more vegetal flavors and aroma, and maybe some fruity / floral aromas and flavors like guava. High oxidation seems to give a bit more of a sweet fruit type flavor, like peach, and sometimes you'll see dried fruit notes (raisin, dried longyan, etc.), especially with aged teas.
2) Look for the amount of red on the leaf. Dancongs and Oriental Beauty are good examples of teas which look dark, especially when dry, but are often medium to high oxidation and very little roast. With a medium roast, high oxidation tea, sometimes you can see these kind of orange-red colors in the leaf, but it's usually pretty hard to see. And, as you say, if a tea has a very heavy roast, it can be hard to tell how it was oxidized based on appearance. But to me, very lightly oxidized teas that are heavily roasted are somewhat lacking in "something" -- for example, a roasted tieguanyin that's well oxidized will often have a nice peachy taste that is hard to get in a lightly oxidized tea that's subsequently given a heavy roast.
3) I think a high degree of "chewiness" of the leaf texture is indication of a high roast, and a leaf that won't unfurl at all would be indicative of a very high (to my taste, often too-high) or not so skillful roast. I have also heard that electric vs. charcoal roast will give different end-results in terms of the texture of the leaf, for example, a high charcoal roast may let the leaf unfurl more than a high electric roast. I am not knowledgeable enough to say how true this is.
4) Roasting and oxidation both bring out a sweet aroma in the tea, if you sniff under a gaiwan lid, or in the empty cup. It's not always easy to tell how much comes from which; however, if you ever stick your head over a tea roaster, or even do a quick refresher roast in a rice cooker, this smell is hard to forget. To me, that's the flavor of the roast. With some teas, even if the tea still tastes a bit sharp, the aroma in the empty cup or under the gaiwan lid can take me back to the experience of sticking my head over a big basket of roasting tea.
5) Look at the color of the brewed tea. In general, oxidation and aging skew the tea more towards orange-red (increasing with the degree of oxidation and / or age), and roasting skews it towards brown (increasing with the degree of roast). But it can be pretty hard to tell for sure just by looking at the color of the tea... it's at least something to keep an eye on, though.
6) A tea that's been roasted heavily, or a lot of times, may have more broken leaves.
Unless you can get a reliable description of the processing parameters of teas you're drinking, it will be hard to know the exact degree of oxidation or type of roast. Also, "high fire" roasting means different things to different people, and a very long roast at medium temperature over charcoal (by a skilled tea maker) will have a different result from a shorter roast at higher temperature on an electric roaster.
