There's a strong, sickly smell: It might sound obvious, but most cannabis grows are discovered by passers-by or keen-nosed residents catching a whiff of the drug's familiar smell.
High levels of condensation: Landlords might notice damp on the walls or peeling wallpaper, while from the outside a neighbour might spot condensation on the windows, even when it's not the depths of winter. The condensation may well be due to the property having been turned into a makeshift greenhouse.
Constantly covered or blacked out windows: Are the curtains drawn all day long? Or have residents placed black-outs over all the windows? It might make it look like the house is unoccupied, but blacked-out windows could well mean that the inside is really bright, with a number of strong lights.
Cannabis growing equipment transported to and from the house: Thousands of pounds worth of equipment is often needed for large-scale grows - have you seen a lot of items being delivered to the house, or large items being taken in or out?
Constant buzz of ventilation: If you can hear the constant noise of a fan, at all times of the day or night, chances are it could be acting as ventilation for the cannabis to grow.
Strong, constant lighting day and night: Cannabis needs light to grow, so watch out for homes with bright lighting at all times of the day and night. Lights will often be on a timer switch, coming on in the middle of the night.
Lots of power cables and rocketing electricity bills: The lights, dehumidifiers and heaters require a lot of electricity. Growers will often hack into the electricity wires before the meter to that individual house, and so bypass having to pay for the electricity. If you are a landlord who gets a copy of the bill, has it dropped suddenly, or gone up suddenly? If you are a neighbour and your bill has rocketed – maybe your next door neighbour has hacked into your mains and you're paying to grow their drugs?
Heat, birds on the roof, and a lack of snow: Cannabis factories produce a lot of heat, which can cause tell-tales signs, especially in winter. When it snows, the roofs of cannabis farms can be obvious as the snow melts, meaning it is probably the only house on the street without a snow-covered roof. Birds also like roosting on a nice warm roof...
Unsociable comings and goings: Are there lots of unfamiliar faces turning up at the house at any time of the day and night? It could just be a popular family, but maybe it's something more sinister.