Do they quote with « guillemets » or leave spaces before punctuation marks ? French.
Decimal comma instead of a period, spaces for thousands separators, or currency sign after the number instead of before it? I'll bet you 5 000,00€ they're European.
„First quote mark aligned to the bottom“? East European.
Group numbers larger than 1000 by twos instead of threes? 1,00,000 rupees they're Indian.
Full width letters? East Asian, likely Japanese.