You can't. You can spear your hands into buckets of sand, try to practice something like the technique by wearing goggles while sparring, and thrust your hands into the air all you want...
...but in a fight, we don't rise to the level of our expectations. We fall to the level of our training. If you never train eye pokes and fingers-to-the-throat pokes in sparring, you won't ever really have a good handle on the technique. Neil Ohlenkamp, judo 6 dan, notes:
I have never seen realistic training in throat strikes or eye gouges in any martial arts class, even though these are often recommended for self defense. The teaching generally done for these techniques helps students to understand what to do, but does not provide effective results for fast, reflexive and accurate application of these techniques against an unwilling opponent in real life combat.
And as Gillian Russell accurately points out,
there are a lot of martial beliefs that we do not get to test in such a direct way. Unless you're unfortunate enough to be fighting a hand-to-hand war you cannot check to see how much force and exactly which angle a neck-break requires, or learn from experience about the psychological effects and stopping power of an eye-gouge.
In an epistemically ideal--though morally horrible--situation, we'd be able to test the effectiveness of techniques by doing them in realistic set-ups over and over again. How many times out of 100 does your no-holds barred nukite to the throat result in death within 5 minutes? 20/10? 80/100? What's the most likely alternative outcome? Bruising? Scratching? Coughing? Unconsciousness? Internal bleeding? Partially crushed trachea? Escalation? Can subjects partially armour against it or roll with it? These questions have answers, but
for good ethical reasons, we can't get at those answers by direct testing, and though martial techniques do get used 'for real', this rarely happens as part of a controlled experiment.
Our inability to properly test the answers to these questions has a knock-on effect. If you can't test the effectiveness of a technique, then it is hard to test methods for improving the technique. Should you practice your nukite (spear hand) in the air, or will that just encourage you to overextend? Is it helpful to practice 1000 a day, or would it be more effective to practice three sets of ten with good focus against a pad? Our inability to test our fighting methods restricts our ability to test our training methods.
(Source - Martial Arts and Philosophy, edited by Graham Priest and Damon Young, Open Court, 2010 - PDF, emphasis mine.)
You'd do better to work on your jab, wearing gloves, against a heavy bag and a good sparring partner. You'll know exactly how well it works. And anyway, as Luis Gutierrez points out, "If you can't even hit a guy with a 16oz. glove how the hell are you going to eye jab him?"
However, with modern gear, some useful practice is possible. You still won't know how much of an effect these techniques will have, but wearing goggles, elbow pads, and open-palm gloves while wrestling is an awesome way to find out where you can punch, elbow, eye gouge, and grab throats or whatever during grappling. You can also study how those strikes and grabs might be countered. Make sure both partners exercise good control, are wearing all the gear, and have full rights to execute the same techniques. Go light.