Is my filling loop valve open or closed is one of the most common questions about boilers.
In the past it was simple question to answer, most boilers had a filling loop fitted near the boiler. If the handle of the valve was in line with the valve it is open if it’s across the valve it is closed. See the examples below.
In the first picture the left valve is open and the right is closed.
In this picture the right valve is open and the left is closed.
You can buy a straight swap replacement filling loop here:
Boiler Filling Loops – Part L Compliant
The question of Is my filling loop valve open or closed is more complicated now as boiler most manufacturers have their own filling loops built into the pipework under the boiler. It isn’t always very obvious which position is open or closed.
Ideal

Ideal boilers (mostly the Logic range) have two blue handled valves under the boiler on the right side.
The valves are off when the far right handle is pointing left to right and the valve next to it is off when it is pointing down.
To fill the system the far right valve is turned toward you and the other valve is slowly turned forward.
Baxi or Potterton
Baxi and Potterton are essentially the same company. They use an integral filling loop connecting the pipes under the boiler on the right hand side. Identification is easy as they are the only blue handles.
When closed the left handle points down and the right points to the side.
The left valve opens by turning it to point backwards. They are normally very stiff if they haven’t been recently. I use a pair of pliers or a small spanner to gentle push it back.
The right valve opens by rotating so it points to the back of the boiler.
They regularly leak when they are used so you can get a straight swap here:
Baxi Multifit CQI Rigid Filling Loop