How can one see if an address is multisig? Some addresses on this page indicate whether the address is a multisig, and what the threshold signature scheme is. How do they detect this?
There are different types of addresses in Bitcoin. The simplest format is called pay-to-public-key-hash (p2pkh) which locks funds to a single private key. Another is called pay-to-script-hash (p2sh), which allows to lock funds to a script that encodes the spending conditions. 

The recipient(s) of a p2sh output must reveal the redeemscript upon spending and must fulfill the encoded conditions to prove their ownership. P2sh addresses are recognized by having a 3… prefix. The most popular application of p2sh is to encode a payment to multiple co-owners, i.e. multisignature addresses. For example in the case of a 2-of-3 multisig, said redeemscript would be composed from 2 pubkey1 pubkey2 pubkey3 3 OP_CHECKMULTISIG where the "2" is the required signatures and the "3" the number of eligible public keys. The proof of ownership is then provided by two signatures corresponding to two of the listed public keys. In other words, once a p2sh output is spent, all that needs to happen is that you read the m-of-n directly from the redeemscript. ;)

Also, since the p2sh address is derived from a hash of the redeemscript, you cannot tell anything whatsoever about the content of the redeemscript until it is revealed in spending (unless you're part of the recipients that created it in the first place).

For a complete walkthrough, see CHECKMULTISIG a worked out example.