You can get a thermometer specially designed for fridges. Ours has a coloured band from 0 degrees C to 5 degrees C, and the fridge is okay if the needle showing the actual temperature is somewhere in that band. The dial setting needed to get that will vary as the weather changes through the year.
You need to defrost it before you can expect it to work properly. When it is defrosted start it up on 4, when the compressor stops humming turn it up to 8, the compessor should come on again, this means that 8 is your coldest setting. If turning it to 1 makes the compressor come on then 1 is your highest setting.
Whatever the dial is set at now, turn it clockwise or counter-clockwise until you hear the motor click on. The direction in which the motor starts is the direction that makes the fridge colder Forget actual temperatures for fridge and freezer. The rule of thumb is if the drinks in your fridge are cold enough to you tasting, and ice cream in the freezer is good and hard, but not frozen solid, that is the setting you want.
My present fridge (a small worktop size one) varies in temperature from one part to another. It has an interesting control too. 1mm further clockwise and it's a freezer making everything solid. 1mm further anticlckwise and the gauge goes into the red 'spoilage' zone. Fun & games eh ?
Larder fridges (those without an icebox) DO ice up on the back wall - that's how they operate. They ice up, then defrost, then ice up, then defrost.....make sure that the channel at the bottom (and the drain hole) are clear and unrestricted.
Also, DO get a fridge thermometer - without one, you are taking it on trust that the internal temperature is correct (3 to 5C).