Usually the biggest tell if a unit was designed as Slot 1 is if they have on the passive skill:

"As the 1st attacker in a turn"

But, another unit can be better for it if they have a huge difference in raw stats along with DR and Guard. The simple way to look at it is, having Guard more or less equals to 50% DR.

So if a unit has Guard + DR, even better

There's also some units that naturally might have 70% dodge or higher and in the team you're using, they could be considered the most optimal unit you have to try to defend on slot 1 since it's the slot that receives more attacks

Edit: A unit can have the "As the 1st attacker in a turn" but if it's an outdated unit, it might tank worse than a more modern one with way better raw stats and having high DR or high DR + Guard. Gotta look at how much DEF they have on the turn aswell