I know of no easy answer. Most Tab does not contain finger instructions (altho some magazines will include it) For chords there are plenty of picture chord books out there. As for playing lead you can decide yourself how you want to do it, i use the following guidelines.....
Usualy there are 2 ways to decide (i assume you are playing lead and not chords) First you have to plan your movement, if you find that your hand is moving large distances in a short amount of time then you have to try and use a different finger for the previos notes. I.E. if you are using your index finger for a note and then have to go down the neck you may want to try to reach it with your middle finger or your ring finger so that the index is closer to the next note you will be playing. The same is true going up the neck, if you are about to use your ring finger for a note then you would rather be using your middle or index finger for the previous one.
Remember, Good lead players aren't exciting to watch because they have alot of movement, if you are continualy rearranging and flying around, it may look more interesting but you are limiting what you will be able to play.
Idealy you should never use the same finger 2x in a row unless you have an open note thats being played. Once you remove the finger the note does not continue to sound (unless its an open string) and therefore your instrament is silent (except for the overtones and harmonics left behind.)
As far as chords if you are using an unusual chord from the tab then its a good idea to play around with the different fingerings to provide easy transitions. I.E. if you are playing a G to a C then if you use your middle, ring and pinky to play the G, your index can already be in position for the transition.
If you learn to take advantage of the fact that you have 4 fingers open to play then you can gain alot of speed by using all of them instead of just 2.
MM