When I say you shouldn't count on it too much, I mean that you can't be absolutely sure. You really can't.
But some people do have better relative pitch than others - there is some science that suggests it has to do with the childhood, and that something triggered your brain to train this ability at an early age.
Very few people have absolute pitch, and they don't even have to be musicians to have it.
Even your neighbor could have it.
Also: there is a very big gap between
good relative pitch and absolute pitch, so either you have it, or you don't.
Olimar, you probably have very good relative pitch, and you are trying to train it as well, so that is very good for you.
I don't have any knowledge of whether I have a good or bad relative pitch, as I haven't tried to focus on mine yet, but if anything, I probably have a bad relative pitch.
You can never expect of someone to have good relative pitch!
And even though you have good relative pitch - as I said before - you aren't sure unless you have absolute pitch, which very few people have - therefore counting on your relative pitch too much isn't something to bet on.
Example: You are supposed to find the key by hearing the key in a song in A major. But a minute ago, you listened to a song that you
don't know is in B major, and a minute before that you listened to a song that you know is in D major - it is possible this series of key changes can make your mind forget which tone is D etc., and that could mess up your decision. It is a fact that it is harder to remember a tone over a long period of time anyways, and when this is meddled with by songs in other key signatures - well, it is a very big chance of loosing the pitch.
^I don't actually know if this example could be true, but this is how I imagine it to work.
At least with me, that is..
In conclusion:
The only way for someone with relative pitch to be sure, is to have something tuned.
Even without recognizing pitches it is possible to hear the scale or mode a song can be in by hearing the pattern of half steps and whole steps. (I tend to relate modes and scale qualities with emotions. Go figure.)
You are correct of course!, but scales and modes are something completely else.