How do i test that my oven works properly and gets up to the right temp?

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think its on the blink as stuff takres ages so fish finger took 35 mins on 230 today , I remeber doing something with white bread laying it out anf timing it in home ec at school or do i put a thermometer in the oven ( will a forehead one do? thanks

u need an oven thermometer

noa forehead one wont do.... they go up tyo 40 C not 230C

i used a confectionery thermometer. Turned out my oven was lower than it should have been, which would explain certain failed cakes...