Before I start, I want to make it clear that I am in no position to claim that the following information is accurate in any way.
From my experience with towers of all sorts once you've climbed a couple, they become repetitive and the only towers that still interest me are spires on top of buildings. The three things I look for when climbing or scoping a radio tower are as follows:
1. Height. The taller the tower is, the more power will be going to it. I generally stay away from anything over 100 meters as it almost certainly has some 80-100kW TV antenna array somewhere near the top. This being said, I have seen towers with nearly no antennas for the first 100 meters so it would probably be fine to go up that high and back down. I've also heard of people (mostly base jumpers) dealing with this issue by only going up 1/2 or 2/3 of the tower. This matters less if you're a base jumper since you don't need to worry about the time you would have to spend climbing down and exposing yourself to ridiculous RF levels.
2. Signs. This only really affects AM sites, but if you see a sign that says AM radiation I would recommend that you not climb that tower unless you have some certain way to tell that it is off and will remain off for the entirety of your time on site. Generally speaking, if I see any sign besides the generic "No Trespassing", "High Voltage", or "Dangerous RF levels beyond this point" signs I will give up on that site immediately.
3. Building size. This is not always an accurate indicator of danger but, on FM/TV towers, the size of the building or even compound near the base of the tower should reflect how much power is going to it. This is not the case with AM towers as some may have a small hut capable of delivering 10's of kW's of power and some may not have any building next to the site. It is also possible that the tower have RF amplifiers/transmitters on it's structure although this seems to only be the case with cell towers.
Some more random bits of information:
On Canada's Spectrum Management System, it is possible that a specific tower will not show up. I have found that if a tower is solely there for internet or cell service, Industry Canada will not bother to register it. In this case you can use some other websites such as this one
https://www.ertyu..../cancellsites.html to find specifications such as who owns it, and how tall it is.
Cell towers are generally safe to climb since they operate at relatively low powers (~100W) although they become worse when you have an array of 18 cell antennas on a tower. Even though these antennas may be deemed safe I would recommend you never go to the top of any radio tower unless it is offline.
Never go in front of an antenna as that is where it concentrates it's radiation. Also, never take a break behind an antenna. I try to rest somewhere off to the side of any antenna regardless if they are designed to not leak RF backwards. The worst antennas are the ones that look like drums because of their high energy density in front that will ruin thin tissues like retinas and potentially cause sterility in males (although I believe it is temporary).
The general risk with these towers besides falling, electrocution, and legal action is the RF radiation heating up parts of your body in a way that the human body cannot handle.
DO NOT QUOTE ME ON THE FOLLOWING (feel free to correct me)
This happens because the radio waves will transfer energy through particles at intervals determined by their wavelength, which is related to their frequency. The lower the frequency, this longer the wavelength.
Drum antennas usually operate at very high frequencies which allows for great bandwidth but those frequencies are incapable of going through most objects. The very high frequency means that the waves will not penetrate very deep into the human tissue before transferring their energy through a particle which will cause that particle to heat up and the wave to lose most of it's energy once it is re-emitted. This means that the antennas will be very effective at heating up human tissue near the surface of the skin or through thin parts of the body such as the scrotum and the eyes.
Cell antennas do the same although their low operating powers don't make them a risk unless you are exposed for a long time.
FM/TV antennas are the scariest since they penetrate very deep within the skin, heating up your tissue deep within your body where there aren't many nerves. This means that you won't feel the RF energy burning your tissue until it's a really nasty burn. This document is good at demonstrating the power of these antennas:
http://www.rfsafet...smoking%20suit.pdf. It's about a climber who was on a tower when a couple of high power arrays turned on. The climber only noticed this when his RF suit started smoking and he had a burning sensation in his leg.
This is essentially all of my experience so far and I hope it helps.