As said in Péter Szilágyi post DAO Wars: Your voice on the soft-fork dilemma:
Miners supporting the DAO soft-fork can do so by starting Geth 1.4.8 with --dao-soft-fork. This will cause the block gas limits to be lowered towards Pi million until the deciding block 1800000 (approx. 6 days from now) is reached. If the gas limit of this block is below or equal 4M, the soft-fork goes into effect and (all updating) miners will start blocking DAO transactions that release funds.
How does the Geth (or Parity) client knows the current global network gas limit ? I can't see any explorer that shows it except https://ethstats.net/ that only shows some voluntary nodes.