Another way to be in tune is to focus on the fretboard. Develop your ability to precisely move the bar to the fretline.
One technique that I think Joe Wright suggests is moving between intervals – start with a semitone, then a tone then a minor third, all the way up to an octave and then back down again.
It is important not to rush this and to be able to have a clear idea of when you will land on the target note.
Using a metronome and aiming to begin and end the slide on a beat is helpful here.