Official Ruby Inspector

Ruby 2.0, 69 bytes

#!ruby -Kn0rdigest
p'×ñF<ìX‚ɲŸ_'.index Digest::MD5.digest(gets)[0]

Hexdump (to faithfully show the binary data in the string):

00000000  23 21 72 75 62 79 20 2d  4b 6e 30 72 64 69 67 65  |#!ruby -Kn0rdige|
00000010  73 74 0a 70 27 d7 f1 46  3c 1f ec 58 82 c9 b2 9f  |st.p'..F<..X....|
00000020  5f 02 27 2e 69 6e 64 65  78 20 44 69 67 65 73 74  |_.'.index Digest|
00000030  3a 3a 4d 44 35 2e 64 69  67 65 73 74 28 67 65 74  |::MD5.digest(get|
00000040  73 29 5b 30 5d                                    |s)[0]|

Explanation:

1. The -Kn option reads the source file as ASCII-8BIT (binary).
2. The -0 option allows gets to read in the whole input (and not just one line).
3. The -rdigest option loads the digest module, which provides Digest::MD5.
4. The code then does an MD5 of the input, takes the first byte of the digest, and gets its index in the given binary string.

CJam, 27 23 bytes

F7EC5ZV4DI8G6]qBb67%J%#

Convert base 11, take mod 67, take mod 19 of the result then find the index of what you have in the array

[15, 7, 14, 12, 5, 3, 0, 4, 13, 18, 8, 16, 6]

Magic!

Try it online.

Julia, 90 59 bytes

Definitely not the shortest, but the fair maiden Julia takes great care in the inspection of the royal rubies.

s->search(s[vec([18 10 16 24 25 26 19 11 9 15 32 34])],' ')

This creates a lambda function which accepts a string s and returns the corresponding ruby defect number. To call it, give it a name, e.g. f=s->....

Ungolfed + explanation:

function f(s)
    # Strings can be indexed like arrays, so we can define d to
    # be a vector of indices corresponding to potential defect
    # locations

    d = vec([18 10 16 24 25 26 19 11 9 15 32 34])

    # Check the specified locations for defects, returning the
    # defect number as the index where a space was found and
    # was not expected. If no spaces are found, 0 is returned.

    search(s[d], ' ')
end

Examples:

julia> f("  ___
 /\\ /\\
/_/ \\_\\
\\ \\_/ \/
 \\/_\\/")
2

julia> f("  ___
 /\\_/\\
/_/ \\_\\
\\ \\_/ \/
 \\/_\\/")
0

Note that backslashes have to be escaped in the input. I confirmed with @Calvin'sHobbies that it's okay.

Let me know if you have any questions or suggestions!

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Edit: Saved 31 bytes with help from Andrew Piliser!