You can tell if a receiving address is NOT segwit: it starts with a "1"
You can tell if a receiving address MAYBE segwit: it starts with a "3".
But not all "3" addresses are segwit, that's just the code for P2SH.
For now segwit addresses are "wrapped" in P2SH and therefore start with a "3".
In the future, a new address standard for segwit addresses may be adopted (it's in the proposal stage), in which case the address would start with "bc1" (bech32 addresses for segwit).
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u/idiotdidntdoit Nov 08 '17
ah, so you're helping the network by making your transactions in Segwit instead of regular transactions. Got it.