There isn't outside of tracking it yourself with like a spreadsheet.
Assuming you have free campfire slots, if everyone in your party has already completed Task 18 (or is otherwise currently on a task), you'll just get a task for a random merc that hasn't instead if you encounter the stranger.
they never bothered turning Vol'jin back on via Stranger, but you can still get his tasks by daily random
There isn't outside of tracking it yourself with like a spreadsheet.
Assuming you have free campfire slots, if everyone in your party has already completed Task 18 (or is otherwise currently on a task), you'll just get a task for a random merc that hasn't instead if you encounter the stranger.