There is one practically fool-proof tactic: make the room you want to feel safe in perfectly dark. Even the highest-end spy cameras need some light source to amplify and capture an image. This means unplugging any electronic with a dim LED 'On-Light' or clock. After you've made the room dark, allow your eyes time to adjust to the darkness and check again for light sources. Windows without blackout curtains will need to be covered with blankets: taped-sealed, possibly (blue painter's tape or Frog Tape will prevent leaving marks or peeling paint on most surfaces). Cracks around the door(s) likewise covered (possibly using the aforementioned tape).
At this point, only one thing remains: check for night-vision/infrared cameras. You can find NV cameras with your own NV camera. See, NV cameras and goggles are by themselves useless in a pitch-black room. That's why most of them come with an infrared light source. Human eyes cannot see infrared 'light' at all, so an NV camera with a built-in infrared beam requires a tool for you to see it. The most discreet option is to bring an infrared-capable camera, as they look like any ordinary camera a person might own who considers themselves an amateur photographer. You can then see beams of infrared light as shown in this video:
Follow the light to its source and disable or cover it. Remember to turn off your camera afterwards, as it will then be a light source for any cameras you missed.
The main flaw with any attempt to try and have privacy while out in the world is that thermal imaging exists and is much harder to avoid. The differences between NV and thermal can be seen here:
This technology can see through uninsulated walls, windows, doors, etc. So even if you perfectly scan your room and find every camera, a person could be tracking your heat signature. You can avoid this by wrapping yourself up in a blanket when changing. Also, if you need to bathe, you can turn the shower on as hot as you can stand it with the windows closed and the fan off, filling the room with steam. Alternatively, take a hot bath.
You are wise to doubt the detective kits, as they can only find cheap, lazily hidden cameras. Even higher-end detective tech, like in bankai's reply, only goes so far. Professional spies use cameras that have built-in Faraday cages, making them virtually unscannable by devices searching for electronic signatures. They, of course, tend to have the weakness of only being able to record without transmitting signals; otherwise, they would then be detectable again. However, spies could then use microwave burst technology to send data packets remotely on a timer, meaning they'll statistically only transmit when not being searched for by an adversary. The KGB allegedly has used X-ray and possibly gamma-ray camera tech to see through walls, as well, and this is detectable with a Geiger counter or similar device.
