You must do it with a computing device because it requires a hashing algorith.
A 12 word seed is storing 12 11-bit (0 or 1) sequences. The first 128 bits are randomly generated. The last 4 are the checksum generated by a hash of the 128 bits. Each 11 bit sequence corresponds to a single word in the wordlist.
You can take a hardware wallet and attempt to restore the seed, trying the last word over and over again until it renders a valid seed.