having good ears has nothing to do with being a good mapper, or player
in short, you should be safe if you roughly follow
help/wiki/Guides/How_to_Time_Songs. throw a couple of notes on the grid, tap them, see the difference. given that you are able to hit these accurately and have good accuracy in general, it should be possible for you to move the offset and see hit error gradually decreasing with every new try. if the error rate stays low for a good time, say, a dozen of seconds, it's safe to say you've found it -- at least the local offset, if there is only one and the composer hasn't mastered the song poorly.
you probably notice that's a lot of ifs for such a short paragraph, and yes, it's not always straightforward. if you need more details, please go watch pishifat's tutorial on timing.
and yes, do not attempt to do anything on 25% playback speed, it distorts the audio, and your results will be inaccurate.