I think it's actually a good question. I am an 8-fixed 4w5, and this made it more or less impossible for me to see myself as either a 4 or an 8 for many years. I didn't know WHAT I was. I'm probably not what most people would think of as a "typical 4" (including myself, bogged down in stereotypes as I was).
As far as "sadism" and "masochism" I've done a bit of both and am actually more identified (in layman's terms) as being "sadistic"--I was the mean older sibling, for one thing.
With all due respect to those who work on this issue, I don't think the answer is as simple as a dichotomy:
I tend to fantasize about power-building while priding myself on being able to cope with reality. I have a very physical edge to me and have occasionally reacted by drawing blades and fists. I secretly suspect I'm horribly broken and flawed, but also obsess with being taken advantage of. Indeed, the two are interlinked--showing how inherently messed up I am will leave me at the mercy of a brutal world...but then I feel like I should stop bullshitting everyone and just be honest about it.
So whenever I start mulling dichotomies over, it's like...Um, both. Kind of. Neither. I dunno.
As I'm sure you know by now, DJ, it really is more about motivations, connections, and focusing on the worst of one's psychology, which, unfortunately, can't be reduced to a simple one-line litmus test.
I personally know I'm not an 8 because I've got the co-motivations of my 3-wing and 5-wing as well as a giant 1-connection. So, one measure you can use is to try to detect the motivations of the wings.
Secondly, and probably more importantly, you want to look at the core, lifelong issues that the individual has had. As often as I've been a commanding asshole to those close to me...as warlike an outlook on life as I've privately maintained...as much as I like a challenge...and as physically angry as I can sometimes be...the core passion that gets under my skin is that of Envy. I keep coming back to the feeling that people all have been privileged in some way that I haven't (usually involving beauty and social skills) and this enrages and depresses me. My first and worst tendency is to be "withdrawn" rather than "aggressive". I feel like the environment has nothing to do with me, rather than being subordinate to me...and I often forget I'm not watching TV.
I hope that's somewhat helpful, although I'm eager to hear @Animal 's testimony, as I suspect she's a lot more 8ish than me.