This is mostly right.
Password strength is measured by how large the search space is for the given password (assuming every password in the search space is equally likely - more on that later on). The search space for a one-digit password is very small {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} and a brute-force attack (trying out every combination) by a human can find your password in about five seconds. A fast or purpose-built computer can do it in under a nanosecond.
A password consisting only of digits that is 100 digits long provides a much larger search space. There are 10^100 possible combinations, so a brute-force attack will, on average, need to try half of these combinations to find your password. This will take a very, very long time, even if you have a computer that can try a million combinations every second.
However, if you choose your 100-digit number non-randomly, e.g. if you prefer a 100-digit combination that consists of alternating 1's and 0's because that's easier to remember, the large search space won't protect you. An attacker who takes human behavior into account will program his computer to try combinations such as this one first and therefore find your password much much sooner than if you'd chosen your 100 digits randomly.
So the process by which you arrive at your password is relevant. If you don't use a truly random process, chances are that your password will be weak and easily cracked.
If you give me a password to look at, sometimes I can determine that it's a bad password at once. If your password is "123456", or "password", or any other of the most common passwords people use these days, it's obviously a very bad password, and I can determine it by looking at the password alone, simply because I see that it's not random at all. But if you give me something like "15-L-s-04-02", it's harder for me to determine whether this is a strong password or not. If you have a daughter called "Liv Sarah" which was born on second of April 2015, the password is very weak if we're dealing with an attacker that knows and targets you specifically, even though the search space isn't that bad (digits, a special character, lowercase and uppercase characters, 12 characters long -> search space of size 2^73, or a password strength of 73 bits, which isn't very good, but still fairly decent).
So: bad passwords can be determined by simply looking at the passsword. It's much harder to determine whether a password is good by simply looking at it, because it's important to know whether it has some kind of meaning for you that another person might ultimately guess.
Still, we can usually determine whether a password is good or bad just by looking at it because we know most password habits of people, so even if I don't know your daughter's name or birthday, I still know that lots of people use names, initials and date components as parts of their passwords, so I'd program my computer to try a few thousand most common passwords first, then have it try combinations with date components or initials, then shuffle these components around in different combinations etc, and the "15-L-s-04-02" would still be found much, much earlier than if I'd just tried every possible combination of 12 characters.
