Directional signs and/or road markings show which routes take you into the charging zone and which don't.
Some approach lanes to the charging zone are also marked with a white symbol.
You will often see red C road markings close to the entry of the zone.
As you leave the congestion charging zone you'll see this sign showing the end of the zone.
On the main approach roads two types of sign show hours of operation and some also show how much you have to pay.