Shiki was an emperor before Shanks but he left the scenes at the same time as Roger by being sent to Impel down by Garo and Sengoku, what happens to him next is a mix of canon and non canon but either way by the time he is out Shanks is already a well established emperor.
What Shanks did noteworthy before becoming an emperor were his duels with Mihawk which impressed even WB, so that probably is already a big piece of his promotion to yonko status. Maybe it could just be that and that explains both his status as an emperor and Mihawk as strongest swordsman (and shiki) with a bounty of 3 B. Afterall in Naruto too the 3 sannin were awarded their titles by surviving Hansoo by the guy himself (the salamander legendary guy, later got killed by Pain).
When he lost his arm he was not yet a yonko (if I am not mistaken) but it seems he already had insane speed (reach Luffy and the bandit in the middle of the sea in an instant, Oda underlined the fact, it repeats in Marineford) and his speciality was already coc. Also lowkey by saying to Luffy he will give him back his hat when he becomes a "great pirate".. which is the nickname for emperor already implies to a degree that he already thinks he has the title in the bag.
And then we come to know he is also called the CoO killer which either means he denies the enemy from using it or it means he can discombombulate people like he did with Killer (basically having a good coo coupled with speed and so totally countering the enemy). Could this be an hint of something? Maybe this just means the one he actually beat to get the emperor title was none other than the invincible Katakuri (jk).
Maybe the navy was just desperate after containing Mihawk marine hunting activity and recognized Shanks as his equal by giving the title, weird but could run too I guess.
Then he was also recognized by Kaido, he put Shanks at the same level as people like Xebec, primebeard, Roger and Oden who gave him PTSD. So of course Kaido knows Shanks power and that is most likely because he fought him... But that is probably just before Marineford so it doesn't count for Shanks getting the title.
So I have no more bullets, imo there is no clear indication nor clear hints about this, maybe there never will be.