Need advice, blown head gasket?

My 1990 celica all trac has been parked in the garage for the past 3 years since buying a second vehicle. The car ran fine prior to parking and has been turned on occasionally every of weeks. About two months ago I noticed that the coolant level had dropped and that the oil on the dipstick had turned into a chocolate milk mixture. I'm pretty positive that the head gasket has blown. I have no major plans of doing a tear down in the near future... What kind of tests can I administer to make sure it is the head gasket and not something else? Should I avoid running the engine at all cost? Should I drain the oil and coolant until I fix the issue? Thanks guys.

I have replaced a few headgasket in my Cressida sedan and Wagon. 

Per these threads, it isnt that big of a job. 

https://www.alltrac.net/phpBB2/viewtopic.php?f=10&t=51411

https://www.alltrac.net/phpBB2/viewtopic.php?t=44663

Either take it apart immediately, or drain all the fluids now, refill the crank case, spin to oil pressure with plugs out,  and fill the cylinders with motor oil through the spark plug holes, then crank it slowly several revolutions with the plugs still out. Do not spin it with the starter after oiling the bores. That is your best hope to preserve it till you can do the gasket.

What would happen if I just drained the oil and coolant and left as is?

Your only guaranteed option is to fix it or unload it now before further water damage takes it's toll.

You have to cook the water out of the motor but you can't do that without fixing it first.

It could be a front cover leak if it is a chain driven cam.

Damage likely caused by electrolysis due to coolant degradation.