I have a rental car and I'm not sure whether I drove through the zone over the last few days. How can I know whether I'm obligated to pay the charge, and if I am but have not done so, then what will happen?
In the UK, the road sign with a white 'C' in a red circle denotes a toll or charge for entering a particular area or crossing a bridge (the 'C' sign is not the only type, but is the most common -- in London, you may also need to pay the Ultra Low Emission Charge, signposted by signs saying 'ULEZ' in a circle, depending on the emissions certification of your car).
To establish whether you entered the congestion charge, ask yourself whether you drove past the relevant signs. Below are some examples to help you.
Entrances to the London congestion charge zone are marked clearly by road signs with a white 'C' in a red circle on a white background:
They usually paint the relevant lanes of the road as well, just before you are about to enter the zone:
Navigation/direction signs also indicate if a particular turning/direction is about to enter the congestion charge zone:
In the above image, the left turn (to Westminster) enters the congestion charge.
By the way, they do give you advance warning on many major routes
(usually, there is a blue background on such signs... not to be confused with the use of blue background to indicate motorway status):
(notice also that it gives advice on how to avoid entering the zone)
Paying the charge
Transport for London's charges and payment options can be seen at:
To get the cheapest available rate, you need to be pretty quick in paying (sorry, cannot remember the deadline), otherwise the price goes up.
Have the cameras recorded your plate?
If you've driven into the Congestion Charging zone there's no way to find out if your vehicle's number plate was recorded or not, other than wait to see if you get a letter or fine through the post.
If you are unsure, I would suggest paying the charge for the days you might have driven into the zone - given you are referring to the weekend, that means you might have until tonight (Wednesday) UK time to pay the charge without penalty. The alternative is a penalty charge significantly in excess.
This map shows the Congestion Charge zone.
If you have driven inside the zone the Automatic Number Plate Recognition system (ANPR) in force will most likely have identified the car you were driving.
The main page for TfL London's Congestion Charge can advise you on how to pay the charge.
These are the rules for when to pay:
1.11 Paying the Charge - payment card
1.11.1 You can purchase a CC Charge at any time during the 64 Working Days immediately before the Charging Day concerned and until midnight on the third charging day after the date of travel into the zone.
If you do not pay, you will be issued a Penalty Charge Notice (PCN). Since you rented the car this notice will be addressed to the rental company which, in all likelihood, will subsequently pass the charge onto you.
If you have a look at the charging zone map you might be able to rule it out if you were nowhere near. If you were using a satnav, it should have warned you that you were approaching a toll road.
Please be aware that there is also a much wider Ultra Low Emission Zone that operates 24-7 in London. If you were driving a newish rental car that should not be a worry, but if the car is diesel powered it must be the latest Euro emissions standard.
If you decide or need to pay, be careful to pay on the genuine Transport for London website. It can happen that an internet search will rank a third-party site higher. It's not so much that they won't pay it (that's fraud), but they will make a surcharge for the service. It is just as easy for you to pay it directly.
There are other toll roads that have an automatic system too, such as the M25 Dartford bridge/tunnel crossing the River Thames.