Yeah, looks like you'll need to take them out of the box. They should have some sort of way to determine impedance, either by the model number, or labeled outright. If by some off chance they don't have any way to find out, just take a meter and measure the resistance across the coil. An 8 ohm sub will measure about 7 ohms on the meter (give or take). A 4 ohm sub will measure 3 ohms, give or take. The DC resistance is usually lower than the AC impedance.
The amp you choose will depend on the coils of the subs. Sub coils can be single coil 8 ohm, single coil 4 ohm, DVC (dual voice coil) at 2 ohms each, DVC at 4 ohms each, etc. Once you find out if each sub is a single coil or DVC, and what the impedance of each coil is, let us know and we can suggest an amp.
No matter what amp you get, you really need to get a box that is designed for those subs. A properly tuned box can make a huge difference in how those subs sound. However, don't expect huge low-end out of 8" subs - they will tend to be "punchy". If you like a lot of loud, deep bass, get a bigger sub (12"). Otherwise, those 8's should do nicely.
Once you get the sub specs, you can use a box design program like WinISD to figure out the optimum box for those speakers, the sound you want, and the space you have. Or, post the TS parameters and I (or someone else) can calculate the box size values.
