Speakers do have ‘polarity’, in a way, although whether or not you’ll care depends on various circumstances. For headphones, for example, it’s fairly important that you do get the polarity the same (whether correct or incorrect, as long as they’re both correct, or both incorrect) as otherwise your brain starts playing tricks on you. For room speakers it becomes less of an issue, although in some circumstances you can get cancellation effects going that, again, make things sound odd. So it can’t hurt to at least wire the speakers up the same way.
You do run into some problems if you have dissimilar speakers and aren’t sure which pin is which. One trick you can use on larger speakers is to just apply a small DC voltage (an AA battery, for example), and when the speaker cone is pushed outward, note which battery terminal (positive vs negative) you applied to which pin.
I don’t think you’d be able to easily use that method on this thin speaker (the deflection would be quite minor and difficult to spot). However, you don’t have to. On this speaker, the + is actually marked on the speaker itself. It’s not easy to see and looks more like just a thick line at one of the solder pads, but it’s there :)