Look out for signs at the boundary and side of the road to see where LEZ applies.

At all LEZ entry points there are signs like these.

Inside the Zone

Once you're inside the LEZ, on main roads there are signs to remind you that you're in the zone and that the scheme is enforced by cameras: 

Advance warning signs

On major approaches to the M25, advance warning signs like these indicate the position of the LEZ boundary, allowing you to choose to divert onto the M25: 

Additional advance signs

Closer to the boundary itself, some approaches have additional advance signs to indicate a route you may take to avoid entering the zone: