How can you tell which one is which?

Well, put simply: an automatic fires as long as you hold the trigger, like the Minigun.
Non-automatic fires only once, like the 10mm Pistol.

I ask for the rifleman and commando perks.

How does the fire-rate weapon stat then come into it?

Well, if it makes any sense, it doesn't. Only thing that should matter is how it fires.

Also, does each rank in Gun Fu override the previous one or are they additive?

Have you ever fired a fully automatic weapon?

Really? Wow.

An automatic weapon shoots its rounds rapidly when holding down the trigger. A non-automatic is single shot, in which you must pull the trigger every time to fire off one round. Both have their advantages and disadvantages.

Single rounds in an automatic do less damage on their own, but when fired rapidly when holding the trigger deal a significant amount of damage and can kill enemies fairly quick. The downside being this can eat through your ammo fast and takes a toll on a weapons range and accuracy, with the addition of more recoil because you're firing consistently. Short burst when firing can help against some of these down sides.

A non automatic round deals a great amount of damage on their own so you won't have to put as many bullets in an enemy and being single shot makes it more accurate and better at range. However, they lack the rapid fire rate of automatics and there are a lot of situations where one shot ain't enough.

Both have their uses in different situations, but it is entirely possible to have a preference of one over the other and use only one of them.

An https://en.wikipedia.org/wiki/Rk_62 like nearly every Finnish man. What does that have to do with anything, though?

Ok, an AK variant. Nice weapon.

Since you are experienced, then you know that firing it full auto requires a slightly different technique than if you were firing semi-auto, if you don't intend to waste most of the ammunition.

While I would agree that many of the firearms perks don't make a whole lot of real world sense, separating semi-auto and full auto does. Although, BGS seems to have failed to take into account select fire weapons like your RK-62.

Ok, but i'm still unclear how that is relevant. Only thing the game cares about with those perks is if it shoots once or multiple times