How will I know if I'm the leader of the guild?

Unless you are paying no attention at all, you'll know. It's made pretty clear.

Your avatar is most disturbing.

You're the guild master once you complete "Under New Management"...which requires you to complete all of the previous main quests of the TG questline, as well as all of the city influence quests that get unlocked when you do a certain number of radiant quests from Delvin or Vex in a particular hold

There will be a ceremony.

And then you're awarded the Guild Master armor.

I know those quests take forever, so I completed all of them and did the special quests before meeting with the Argonian in Solitude.

I've now done 25+ side jobs (mostly for Vex) and completed 2 City Influence quests!

So the Flagon now has two new merchants I've never seen before...

I just keep hammering away at it, and won't take the Skeleton Key back until I make further progress.

But I'm Level 46 now, and haven't even been to Winterhold/Morthal/Dawnstar etc. (let alone Dawnguard/Dragonborn)!

Tick tock...

But finishing the main TG questline without the official Guildmaster ceremony (and armor) feels kinda... shallow.

So this time I'm doing it right!

(I just hope I live long enough, heheh... it's plenty time-consuming)

Finish the questline for thieves guild

you can cancel jobs.
you need 5 jobs in whiterun, windhelm, solitude and markarth. just keep asking for a job and cancel it if it isnt in one of them.

Or save and load. It takes longer, but it doesn't count as a failed quest. I stopped giving a [censored] about pointless stats like that a long time ago, but just in case you do, best you know. I was kinda surprised in one of my earlier files when I found out I had -32 thieves guild jobs completed as Guild Master.