Unknown name joined your koko and you are unsure whether he's from /mjg/ or /jong/?
Now you can easily answer this question by consulting THE LISTS.
/mjg/ players https://files.catbox.moe/ahi274.txt
/jong/ players https://files.catbox.moe/cas60u.txt
Number indicates how many times they were seen playing in a room posted in their respective thread. Note that low values may be caused by /jong/ rooms being reposted to /mjg/ and vice versa.