I recently received an E-Flite 800mah lipo battery in trade(from a CX2 heli). And it just so happened that when I went to the hobby shop the other day, they had the exact same charger that came with the CX2 heli meant for this battery on sale for $10 bucks so I picked it up.
[link=http://secure.hobbyzone.com/catalog/HZ/eflite/eflite_blade_cx2/eflite_blade_cx2_parts/EFLC3110.html]Charger[/link]
[link=http://secure.hobbyzone.com/catalog/HZ/eflite/eflite_blade_cx2/eflite_blade_cx2_parts/EFLB0990.html]Battery[/link]

I am running this battery on a Mini-T with a Mamba 25 6800 setup. It gets about 2-3 mins runtime before it hits the 6v cutoff I have the ESC set at. So then I take it out and hook it up to the charger. It has a solid red/green light meaning it is charging. But then about 10 minutes later it goes to solid red which means it is fully charged. I mean correct me if I am wrong but the charger only charges at .65 amps so in theory it would take an hour to charge a 650mah battery....right? So how is it saying that my battery is fully charged/balanced in less than 10 minutes.

Could the battery be defective or worn out, or is the charger defective? Is there any way I could test either of these to figure it out?

It could also be that the battery is never getting empty. Instead of operating on the bottom portion you may be operating on the top portion , so to speak.

To tell if it is full check the voltage, 4.2 per cell is full charged and about 3.7 is empty. (resting voltage)

If it is hitting the 6v cutoff then wouldnt it be reaching 3v per a cell? Sorry if I am asking dumb questions, but I am new to lipo and I dont want to mess anything up considering I have seen the videos of what can happen to misused lipo packs. I just dont understand how the battery can get 2-3 mins runtime and go from hitting the low voltage cutoff to "fully charged" in under 5 mins each time(I charged it a couple more times and it cuts off around 5 mins +- a min every time).

Yes, but that is under a load. The voltage sags under load and depends on current, connectors,leads,load, ect.
Try a resting voltage check after it hits the cut-off and then after it is charged.

It may very well be hitting the cut-off yet the battery is still more than half charged. In fact it is probably over 80% still charged. The two voltage checks will tell the charge level very closely.