I just bought a 1986 El Camino Last night. The VIN is 3GCCW80HXGS913299. I found a VIN Decoder on the internet that said this was a "El Camino SS". Is that true?

Is there a way I can tell what my rear end gears are (other than pulling the cover and counting the teeth)?

How do I tell what kind tranny the stock 3 speed auto that came with it is?

There should be numbers stamped on the front of the axle. If you can't find it, then you might want to pull the axle cover and count them manually. just put jack stands under it and rotate the wheels. It is pretty easy and you can replace the gear fluid any ways. All you need is some liquid gasket stuff to seal it back up.
If it IS an SS it probably came with some pretty stout gears. You'll want to take care and put new gear fluid in it anyways.

your trans is a TH350

...I found a VIN Decoder on the internet that said this was a "El Camino SS". Is that true?
The VIN will not tell you if it's an SS. Look at the RPO label on the inside of the glove box door. If it has Z15 as one of the codes on it then it's a factory SS.

...Is there a way I can tell what my rear end gears are (other than pulling the cover and counting the teeth)?
The eighth letter in your VIN indicates you have a 305 motor. There were only two options for a rear end with a 305. You'll have either a 3.08 or a 2.41 rear. Look for either a GU1 (2.41) or GU4 (3.08) on the RPO label on the glove box door.

How do I tell what kind tranny the stock 3 speed auto that came with it is?Again, look at the RPO sticker. There will be a code that starts with the letter "M". That is the transmission code. List it here and I can tell you what it came with.

Stick with Ernie. He knows his stuff.
Where the hell did you get that cool azz avatar Ernie? That is sick! I love it

Where the hell did you get that cool azz avatar Ernie?
I made it. It's just another one of my many talents.

ELCO SS
The Super Sport package, option Z15, included a large front air dam; matching painted sport mirrors;
special black paint treatment on quarter window trim; choice of six paint accent colors on lower body;
decal stripes accent the paint color break lines and rally wheels. The Regular Production Option (RPO)
for the Super Sport is Z15 and in 1987 a no decal option was offered
The Z15 RPO was furnished with cloth covered bucket seats and a console unless ordered otherwise.

Also most people are unaware of the fact that the 1985 thru 1987 El Caminos and Cabbaleros were made in Mexico. There were none manufacture in 1988 altho several left overs from 1987 were sold as 88s.

There are not stickers in the glove box or under the hood - so there is NO way for me to know. It has the fancy sport mirrors but not a big aire dam in the front although there is one there - so i have no way to know. IT is NOT a TH350 - a receipt from a tranny shop a few years ago lists it as a "200" but i know it is a 3 speed auto and it must have some tall highway gears cause it runs pretty slow off the line and bogs a bit. Any further help?

I think you can just jack Both rear wheels off the ground if it is a posi. if is a one wheel peel job the just one wheel. put it on jack stands and block the front wheels.You know Safety first. Put it in neutral and turn the wheel and watch the drive shaft. you need to count the revolutions of the wheel in relation to the drive shaft. I belive if you where to have say, a 3:50 rear gear the wheel would turn 1/2 a turn and the drive shaft would turn 3 times. I hope this helps.

Have fun and lets see some pics.