My truck is gulping antifreeze but there are no visible leaks anywhere and I haven’t noticed anything unusual out of exhaust pipe.The temp needle is fluctuating and it is running a little rough.Ive been told it could be anything from a thermostat to a fan clutch,intake or head gasket.How can I diagnose on my own? 
does it knock when its cold? IF it does, Id check for broken bolt on manifold.
Check to see if it looks like oil has antifreeze in it, or antifreeze coming out exhaust, if so probably something to do with manifold.
If there is no visible leaks, Id focus on the manifold
Using water and temp spikes indicate a head gskt, or more likely a cracked head. Do you notice a sweet smell from the exhaust?
Top off the radiator with water/anti-freeze and run until operating temp comes up. CAREFULLY open the radiator cap, using a rag or towel large enough to prevent anything blowing past and onto your hands or clothes. If you have a whitish-grey foaming madhouse going on you have a head gasket out or a crack in a head. I’m guessing that as long as it just runs rough you have a gasket out. Those are a common problem.
Sounds like an internal leak, crack in heads or block, and or leaky head gasket.
I have experienced both at one time or other. If nothing comes out the exhaust pipe, check the oil, see if it is milky. This could mean a leak on the lower engine side (block) causing coolant to go into the pan.
I had a Chevy one time that had a leaky heater hose that was hidden under a left side manifold that was leaking and hard to find but was a simple fix after it was found.
A bad radiator cap can cause problems, usually causing the engine to run hot.
Hope I haven’t confused ya.
Roger 
freeze plug leaking?
You did not say what engine my guess is you have a 5.7 if no bubbles in the radiator then you need intake manifold gasket will be the culprit. Done a few in my time. If this is the issue make sure you get the better gasket set, that way when your done your done. The old gaskets would go out again after some miles put on the vehicle.
It sounds like the lower intake gaskets are leaking. The gauge moving around is due to loss of pressure in the cooling system. I’m assuming that they are original. They are Plastic and pulverize over time and will leak internally.This usually starts as a small leak at first and will usually only loose coolant on warm up and cool down.the engine will also start to run poor. When they get really bad they will start to leak externally. You will not see coolant in the oil because dexcool will mix with the oil vs direct seperation which is why gm uses it. My advice is replace the lower intake gaskets. You can leave the upper plenum attached to the intake. When the intake is off look at the cooling ports on the gaskets. They will be sucked in and cracked. The plastic around the ports will be brittle. On the heads you will see buildup at these passages. It is from the stopleak In the dexcool setting up from the leak. There are a couple tests you can do to verify that this the case.
First you can do a block test. It can be purchased from napa. It is a kit with blue fluid. The fluid will turn yellow if hydrocarbons are present in the coolant. This would indicate head gasket failure.
Second. You can do a cylinder leak down test to each cylinder or suspected cylinder and watch the coolant in the radiator for air bubbles.
If the problem just started ,I doubt that a cracked head is the cause. This usually only happens after severe overheat. A fan clutch can cause it to run hotter but not fluctuate all over. A sticking thermostat can cause it to run hot and fluctuate but not loose coolant. Your best bet is the intake gaskets. Easy repair and parts are cheap. Hope this helps. Good luck 
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