You can go here for the (rough) specs for each tuner type:
(rtl_test.exe referenced on that page will test the E4000 version. See here: RTL-SDR Installation Instructions
The plastic case of most dongles can just be popped apart with fingernail pressure.
One of the big chips should be the rtl2832U (Typically the larger of the two)
The other will be the 'Tuner' chip. You should be able to read the number on that and reference the specs.
Normally they're marked in gold/yellow on black and they're hard to read without a good light and lens.
You can also note the stenciling on the pc board to compare with images on ebay and elsewhere.
Edit: You can click on the left thumbnail here (and mouse over) for an idea what to look for.
NooElec - TV301U DVB-T USB Stick (E4000) w/ Antenna and Remote Control - SDR Receivers - Software Defined Radio
I should also mention that those dongles are notorious for bad/cold solder joints.
Especially at the antenna connector. Of my five, two had that problem.
And, to save you searching around, this may be helpful.
RTL2382U - The ADC and USB data pump. It is mated with one of the following three tuner chips. NDA needed for the data sheet.
E4000 - Tuner. 60MHz to 1700MHz The Elonics E4000 is the most desirable tuner to have as it's the most capable. This part may now be in short supply
FC0012 - Tuner 50MHz to 1000MHz ; Fitipower - no public information available.
FC0013 - Tuner 50MHz to 1700MHz ; Fitipower - no public information available.
R820T - Tuner 24MHz to 1850MHz ; Now the E4000 has gone this is the tuner to get as it's comparable in performance. It also seems to be cheaper overall.