2 questions

Yeah, it is a can of worms.
The fact balanced connections started to be a fad. So even stuff which does not benefit from them has them.
When a product has one circuit aand the other side is just a ground. that is single ended. RCA and the majority of equipment is made this way.
IF a sigle ended electronics was to have a 'balanced out" it i s done with a op amp flipping a copy of the single ended signal into reverse polarity, and then sending that out as the negatice side of a balanced connection.
The original is sent out on the plus side.
So it basically doubles the votalge going out the connection.

When the balanced signal gets to the electronics which also is not 'true balanced. The signal is again converted to a single ended one.

So it is true that single ended stuff can have these connections.
Are they better than RCA?
Can of worms opening... Wrrrrrrrr....

PS My preamp is single ended, with balanced out, which i use to my single ended amp, only becasue the wire is 7 meters long.

I own one item which is truly balanced throughout.
An Audio Research PH-2 phono box. which uses dual circuits for each channel, amplifying both the positive leg and negatice leg. So it has twice the circuitry of a single ended box.

I've been using balanced cables for many years in both pro audio and (more recently) hifi stuff. Even if the results aren't truly balanced, they're a better design. Period. They lock into place, they're easier to handle, often have better shielding, you're less likely to touch and contaminate the connections, and I like them...they make me happy. Now if we could get rid of phone plugs and replace everything with Neutriks made in Liechtenstein.

So how do i find out if my xlr's are truly balanced?

No need to find out. Use 'em. For a 5' cable, it will make no difference.