Find a script called bq27200.sh, designed for battery monitoring. Among other things, it can show you the total charge cycle count since last full reset (which is not easily done by average end user). For example on my year-plus used phone...maemo:~# ./bq27200.sh LOOPMODE=0 Reading values...........................done. CSOC: 100 % RSOC: 100 % Average Current: 45 mA TTF: 0 minutes TTE: 65535 minutes NAC: 1425 mAh CACD: 1425 mAh CACT: 1425 mAh SI: 10 mA STTE: 8121 minutes MLI: 1028 mA MLTTE: 81 minutes AR: 0 mA ARTTE: 65535 minutes Last Measured Discharge: 1425 mAh Cycle Count since Learning: 1 Total Cycle Count since last full reset: 293 Reported Battery Voltage: 4156 mV Battery Gauge die Temperature: 35 C Flags: 0xa4 Charge:1 NOACT:0 IMIN:1 CI:0 CALIP:0 VDQ:1 EDV1:0 EDVF: 0 eeprom data: ILMD=2056 EDVF=3000 EDV1=3248 ISLC=20 DMF=14700 nanoVolt SD=201 thousandths of percent per day AGELMD=1 TAPER=102 mA IMLC=1028 mANote the 293 charge cycles. On a phone that I bought off eBay with a (credible) month or two's usage, this count was at about 60. Doesn't give you proof positive, but, if the battery chip shows 10 or 20 charges, it's credibly a "new" phone. If it shows 100 or 200 charges, your seller has some explaining to do...
maemo:~# ./bq27200.sh LOOPMODE=0 Reading values...........................done. CSOC: 100 % RSOC: 100 % Average Current: 45 mA TTF: 0 minutes TTE: 65535 minutes NAC: 1425 mAh CACD: 1425 mAh CACT: 1425 mAh SI: 10 mA STTE: 8121 minutes MLI: 1028 mA MLTTE: 81 minutes AR: 0 mA ARTTE: 65535 minutes Last Measured Discharge: 1425 mAh Cycle Count since Learning: 1 Total Cycle Count since last full reset: 293 Reported Battery Voltage: 4156 mV Battery Gauge die Temperature: 35 C Flags: 0xa4 Charge:1 NOACT:0 IMIN:1 CI:0 CALIP:0 VDQ:1 EDV1:0 EDVF: 0 eeprom data: ILMD=2056 EDVF=3000 EDV1=3248 ISLC=20 DMF=14700 nanoVolt SD=201 thousandths of percent per day AGELMD=1 TAPER=102 mA IMLC=1028 mA