How long does it take for the body to detox inhalants such as car gasoline? Is there a way to test for it through a blood or a urinanalysis?

It is done through blood tests.

It is done through blood tests over a period of months to make sure there is no neurological damage. Recovery could take months depending on how long you have been inhaling.

How far back can they blood test for huffing gasoline

What do you mean how far back? You go and get a blood test that is it.

What chemical do they test for?

It will depend on what they suspect, but most cause lasting harm, so passing a blood test is the least of your worries

Drain bamage, I'm sorry, Brain Damage should be your number one worry, not testing.