Fifths are one the hardest intervals to play in tune, because both notes are usually played with the same finger and adjusting the intonation is not intuitive. It's also very obvious to the listener when a fifth is out of tune!

The two factors that control the intonation are the way the finger is placed in relation to the two strings on the axis between G string and E string, and the angle of the hand, how the hand is rotated. If you get a feeling for how these two factors interact, you can eventually find positions for all your fingers to play fifths in tune quite reliably. It doesn't always work, as you can hear in one of my examples when I was getting hasty while demonstrating and didn't adjust the angle of the hand when I changed strings. As I said, it's not intuitive!
There are always a number of different solutions for each finger, or combinations between position of the finger and angling of the hand, that will get a good result. I myself prefer playing fifths with the firm part of the fingertip, rather than the fleshy part, but other violinists use the fleshier part (particularly if they have thin fingers) and some even play with a collapsed joint. Playing fifths is the one occasion when having thicker fingers is useful!
As you go up the string into high positions, the interval will remain in tune as long as the position of the finger in relation to the strings and the angle of the hand remain constant. As one of these factors changes, the other has to adjust with it.
(This is assuming that your strings are not too old and "false". You can check whether your strings are false by playing high harmonics in fifths - if the open strings are in tune, the high harmonic fifth must be in tune also. If it is not, it's time to change your strings!)

When you encounter a fifth in a double-stop passage, the angle of your hand may be affected by the double-stops around it - if you wonder why you can play a specific fifth in tune by itself, but not in context, this is probably why.
If you watch the video until the end, I play one of the most notorious fifth passages in the repertoire, from the end of Bartók's second violin concerto!

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