Since Julia’s tablet has an eight-digit code and her fingerprints reveal the exact eight digits, we can assume the spy has identified all possible digits for the code. However, he still needs to guess the correct sequence.

1. Determine the number of possible sequences:
   Since the spy knows the eight digits but not their order, all eight digits could be arranged in any sequence. The number of possible sequences of these eight unique digits is the number of permutations of eight items, which is calculated as: $8! = 8 \times 7 \times 6 \times 5 \times 4 \times 3 \times 2 \times 1 = 40,320$

2. Calculate the probability of the correct guess:
   Since only one of these sequences is the correct passcode, the probability that the spy correctly guesses the passcode on the first try is: $\frac{1}{8!} = \frac{1}{40,320}$

3. Express the probability as a decimal:
   Converting $\frac{1}{40,320}$ to a decimal, we get: $\frac{1}{40,320} \approx 0.0000248$

Thus, the probability that the spy successfully unlocks the tablet on his first try is: $\boxed{0.0000248}$

Would you like more details on permutations or any other related concept? Here are some additional questions to consider:

1. What if Julia’s passcode allowed repeated digits? How would that change the probability?
2. How would the probability change if the spy only saw seven digits on the screen?
3. What is the probability of guessing an eight-digit code if the digits are randomly chosen from 0-9?
4. How many attempts would the spy need on average to successfully guess the code?
5. What if there were some smudges, leaving ambiguity about certain digits? How would this affect his chances?

Tip: Knowing factorial calculations can be very useful in probability and combinatorics!