The easiest way is of course to just look on the box. But, this is when you don’t got the box.
You got two options of origin: either Taiwan or China. And they’re both just as good, I haven’t found any differences myself and I’ve used phones made in both countries.
Easy way with just your eyes
Check S/N (serial number) under Settings > About > Phone identity. Or on the box.
First two letters of S/N
HT = Hsinchu, Taiwan
FA = Taiwan
SH = Shanghai, China
SZ = Shenzhen, China
CH = Wuhan, China
This at least the information that I’ve currently got. If there’s more information. Please Let Me Know.
With phones made in 2014 and on (this checks out on my One M8)
FA45Dyyzzzzz Taiwan, 2014, May (5), 13 (D)
With the following structure
FF = Factory, look at list above
Y = Year (4 = 2014)
M = Month (hex 1…C = 1 – 12 months)
D = Day (hex 1…9A..Z = 1 – 31 days)
yy = Part Code
zzzzz : Identification Number (decimal)
Month should be in decimal form, but can be in hex form.
Days will go from 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, G, H, J, K, L, M, N, P, R, S, T, V, W, X, Y, Z. Day Z represents day 31.
Using a site
There’s IMEI info sites which will tell you where/when the phone was made. THIS site gave very accurate information on my phone at least. That page checks warranty and manufacturing date, for more pure IMEI go HERE.