things don't work is the obvious answer.
Not while it is in the fuse holder. Pull it out and if you cannot look and tell if it is blown or not, test it with an Ohm meter.
Remove the fuse and check it with a ohm meter. If you have no ohm meter then just replace it with a new fuse.
You can tell if your heal lamp relay is blown is by inspecting the fuses. If you look closely at the fuse, you will be able to tell it is blown or not. If you see a separation in the fuse, it is blown.
Look at it closely to see if it is blown. If you cannot tell by looking then check it with an ohm meter.
There are many ways to tell if a fuse is blown. The most obvious way of telling is if it doesn't work.
Remove the fuse and look at it. If you cannot tell if it is blown by looking at it then check it with an ohm meter.
A relay does not have a fuse. The circuit for the particular relay is fused. You can tell if the fuse is blown by testing it with an ohm meter.
If the car is a Diesel there is 30 amp open fuse on the firewall
Main fuse.
Most car radios have a fuse on the back where the plugs are. Check the fuse to see if it was blown or smell around the car radio you might catch a burnt smell also
Pull the fuse out and have a look at it and if the wire inside is broken then its blown
Unplug the refrigerator, remove the fuse and look at it. Typically the fuse will be dark in color, much the same as a blown light bulb.