How do you tell if a Li-ion battery has a protection circuit?
The battery is most likely a 3S Li-ion pack, i.e. 3 cells/packs in series. Protection circuits for single cell Li-ion normally have overdischarge protection set somewhere in the range 2.5V-3.2V per cell, which translates to 7.5V-9.6V for a 3S pack. So this is the range that you should test to ensure that the undervoltage protection correctly triggers.

The listed output voltage 10.8V-12.6V is probably meant to be a conservative estimate of the usable voltage range, since there will be little accessible capacity left below 10.8V (except at very small currents). 

Edit To respond to a comment on the question, if you do a search on the model number "DC 12680 protection" you will find other listings (e.g. on Amazon) where it claims to include protection circuitry.

You will also find pages that unmask the typical overinflated capacity claims on these packs. For example, here it is discharged at 1A down to 8.5V and tests at only 25Wh. Notice that this is 3.7V * 6.8Ah = 25Wh, not 11.1 * 6.8Ah = 75Wh. The industry standard is to quote capacity at the cell nominal voltage 3.7V not the pack voltage (here 11.1V). Many unscrupulous sellers exploit this ambiguity to mislead the buyer into thinking that the pack has much higher capacity, here 75Wh vs. 25Wh. Caveat emptor.