For the most part, a radioactive substance in a room can only be detected by using a device designed for that purpose. The most common device is a Geiger counter. It "counts" the particles discharged by the radioactive substance, so you can tell when more of these particles are being released than non-radioactive substances normally release.

What's interesting is that most objects release the particles that we normally associate with radioactive substances, they just release fewer of them. If you hold a Geiger counter up to certain rocks, or even to direct sunlight, it will tell you that there's some radiation there!

Radiation actually affects more than just the brain. There are different types of radiation, classified essentially by how much energy is present in the particles they give off. Particles given off by some types of radiation can burn your skin and other tissues, but some particles are blocked by your skin and only cause damage if they get into your body somehow (you swallow the substance that's emitting them, you get the substance in a cut, etc.). Some radiation particles are so small and fast that they're completely harmless because they pass right through your body and out the other side! One of the biggest concerns with radiation is that it can cause mutations to occur in DNA. That's how radiation can cause mutant animals to be born, like frogs with five legs. That's also how radiation can lead to cancer--the mutation it forms can cause a cell to divide uncontrollably, forming a tumor.

The only way I know to detect radioactivity is to use a radiation meter. I don't know that radiation affects the brain - or the rest of the body - in any dangerous ways at the levels we are typically exposed to.

If we lived in Denver instead of Santa Barbara, we would be exposed to twice as much radiation, because Denver is a mile above sea level. Our bodies are actually very talented at taking care of themselves, and they have many enzymes to repair anything that gets damaged by radiation or other things. There are a lot of enzymes that repair our DNA molecules when they get damaged.

Everything on earth has been exposed to the radiation from cosmic rays for all of time, so it's quite normal to be exposed to some radiation, and you don't need to worry about it.

Yes, the radiation from radioactive substances does affect us all over --- for example, it can cause cancer of the skin. The simplest way to test for radiation is by using something called a scintillation screen. When radiation falls on the screen, the radiation is converted to visible light (like a little green pin-point of light) and this can then be measured.