It would not be smart to not implement SegWit if/when it becomes part of the mainnet.
Why? Segwit brings many improvements to Bitcoin that we need, including scaling. Many wallets have or are in the process of implementing segwit.
Some Bitcoin companies may however not "support" SegWit and would prefer that it not be implemented. My question is, how can a non-mining entity that is part of the Bitcoin economy and/or ecosystem show their support/non-support of SegWit?
Nodes that signal segwit have the NODE_WITNESS service bit set. So to check if a node supports segwit, just check for that service bit.
Even if an entity does not support SegWit they may prepare for SegWit in the event that it becomes part of Bitcoin despite their wishes.
Segwit is backwards compatible. If you don't like segwit, you can continue to use Bitcoin as you do now with almost zero impact on you.
Edit:
It would not be smart to not implement SegWit if/when it becomes part of the mainnet.
Why? Segwit brings many improvements to Bitcoin that we need, including scaling. Many wallets have or are in the process of implementing segwit.
Look closer. That was a double negative.
Hah. Whooops.