There is a Small 11mm Plug on the bottom of the transmission if your looking towards the back of the car when your under it its on the left side of the transmission its a small bolt start your car and take that out if transmission pours out its full if not fill it up till it starts coming out.
There is a thread on checking the ATF in the Cobalt:
No Dipstick...need to have skills as a mechanic...car on lift, engine running, at operating temperature...remove check plug on front of tranny...fill fluid until it comes out of the check plug hole and then replace plug. DO NOT stop engine until the check plug is in place or you will dump hot fluid. Fill through the vent on top of the tranny (plastic plug).
Originally Posted by Maven
(paraphrased straight from the GM manual)
Start the engine and allow the engine to idle until the transmission fluid temperature has reached at least 105degF. Depress the brake pedal and move the shift lever through the gear ranges, pausing a few seconds in each range. Return the shift lever to the PARK range.
Remove the fluid vent cap. (red cap on drivers side top of trans)
Raise the vehicle on a hoist(or support on jackstands)
. The vehicle must be level, with the engine running and the shift lever in the PARK range.
Remove the transmission oil level control plug(its a small usually 10mm hex bolt on the right/passenger side of trans, below and in front of the right hand axle.)
Check the fluid level. The fluid level should be even with the bottom of the threaded plug hole.
Add DEXRON®III automatic transmission fluid in increments of 0.5 L (0.5 qt) until the fluid drains from the plug hole.