Viewing Heat Levels
On the left there's a box called the 'System Menu' from it you can view the stats page.
The Overview tab in this stats page will show the different organisations heat levels.
Viewing FOJ Threat Levels
The Global Organisation tab in this stats page will show the FOJ Threat Level for each organisation. The higher the threat level, the better the quality of investigators are sent.
How Heat Levels Work
If you own a network in just 1 of the 5 areas an organisation owns.
For example... 1 blue area, which is controlled by P.A.T.R.I.O.T, then the heat level for Patriot shown in the stats page will reflect what the map heat indicator shows for that particular area. If you have a network in all 5 areas owned by Patriot, it'll combine all 5 areas.
So let's say you have 1x Patriot area on lockdown (50 heat), 1x Patriot area on 25 heat and 3x Patriot areas on 0 heat, then in the stats page Patriots will show a heat level of 75.
How to Lower Heat Levels
You can use the Purple Schemes to reduce heat either Instantly for gold, or over time using minions.
But remember... lowering the heat to 0 in one area will not lower the total heat for that organisation if you have other networks in their areas.
You can also destroy all the networks you have in areas owned by an organisation to completely stop FOJ investigations by that organisation. Handy if you're being overwhelmed.
How to Raise Heat Levels
Whichever organisation has the highest threat level is the organisation the FOJ send to investigate.
Different islands have different organisation vulnerabilities, for example the 3rd island has low vulnerability to Patriots. But high to ???
It's therefore helpful to keep the Patriots heat/threat levels higher than ???, so when FOJ send investigators, they're Patriot investigators.
From what I can tell, killing or detaining investigators that come to your base also raises heat/threat levels for the organisation that sent them, so Distraction is your best friend.
If you're constantly being overwhelmed by investigators or soldiers, I suspect your heat/threat levels are high.
Unfortunately if they're high to all the organisations then reducing the heat/threat to just one, will only switch which organisations the FOJ sends investigators from.
In this scenario, I'd destroy all my networks except for the Southern Anvil Province (above Australia) as this area has a scheme for 10k gold on a 3 minute timer and an instant heat reduction for 5k.
When the Southern Anvil Province is your only network, the FOJ takes longer than 3 minutes to send an investigation and that investigation is also cancelled completely when the heat level is reduced back down to 0. Giving you an easy 5k risk free profit.