LED Christmas lights without limiting resistor
If you are willing to watch "Big Clive's" vlog, he uses the "easiest" method of finding the resistors, a thermal imaging camera. 

In the 220V set he found 3 resistors in 3 of the LED bases, the rest had no resistor. Each resistor was 510R for a total of 1530\$\Omega\$. He also says he found a bridge rectifier molded into the bit where the wires split.



That was for an apparently Polish made set. It's certainly possible that other sets
use variations on this theme but you get the idea. Bridge rectifier plus a few hidden resistors distributed out among the LEDs (to spread out the heat dissipation, otherwise they could use just one resistor).