Exchanges should be monitoring the situation on behalf of their users and advising them (e.g. by mail) if there is a near-fork situation and what the exchange will do if a fork happens.
At least the more professional among the exchanges will probably communicate clearly.
Monitor their news / blog pages / twitter feed of your exchange carefully.
Can't hurt to check what a variety of the big exchanges are saying - get yourself an overview picture.
However, let's say your specific exchange refuses to make any announcements or monitor on customer's behalf. Then withdraw your coins in good time before a fork happens, because it may take exchanges a while to sort things out - especially if it's a dodgy operation in the first place.
The currently known possible forks are
- Bitcoin Unlimited (BU) hard fork (coinbase + useragent signals)
- Bitcoin Core SegWit (SW) soft-fork (BIP9 - 95% threshold activated)
Both of these forks operate primarily on signaling mechanism that are encoded into blocks.
For (1), there are sites such as
http://nodecounter.com ,
http://bunodes.com ,
https://coin.dance/blocks and
http://bitfire.io that monitor the relevant signaling. The actual activation is not tied to a fixed threshold, but depends on a plan followed by miners.
Currently, this plan is described in [1], but the miners could, together with other Bitcoin industry participants, decide to revise this or follow a different plan. When this happens, it will be widely announced in Bitcoin news media. You might want to follow some Twitter accounts or online cryptocurrency news outlets in order not to miss anything.
There is no built-in expiration timeout in for the BU hard fork activation.
For (2), currently the activation mechanism is described by BIP9. The procedure for SegWit is described in [2]. You can track it live at
http://bitcoin.sipa.be/versions.html
However, there are deliberations for changing to an alternative activation method called User Activated Soft Fork (UASF), because the miner signaling for SW has not lived up to expectations, and is not looking like it will pick up. If SW has not activated by the end of one retarget period after 15 November 2017, it will cease to be eligible for activation.
[1]
https://medium.com/@ViaBTC/miner-guide-how-to-safely-hard-fork-to-bitcoin-unlimited-8ac1570dc1a8
[2]
https://bitcoincore.org/en/2016/10/27/release-0.13.1/
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The above are only the currently declared possible forks.
It could happen that other fork proposals enter the market. Bitcoin news sites should cover them.
