Uh, USUALLY. If uncertain, take an AA battery and connect it to the speaker terminals with some jumpers. Put the + of the battery to what you think is the + of the speaker. If the speaker cone moves OUT (away from the magnet) you've got it right. If it moves BACK (towards the magnet) it's not what you think, it's the other way round.
Despite it being a "convention" for the larger to be +, there are some speakers which will fool you. I always check to be sure when mixing speakers so I don't end up with phasing problems.
Don't LEAVE the battery connected long -- just long enough to tell. If it's a pertty high wattage speaker you can use a 9volt battery which is easier.