Check whether disk was replaced I recently sent my 2012 MacBook Pro to a technician to repair it and he said he replaced the disk but I do not believe he did so. Is there any way I could prove he didn't?
You can check the S.M.A.R.T. status of the disk with a tool like smartmontools!

To get the details simply enter smartctl -a disk0. You may have to enable S.M.A.R.T. with smartctl -s on disk0.

Example output of a 3yo 500 GB SSD:

...
SMART Attributes Data Structure revision number: 1
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x0000   100   100   000    Old_age   Offline      -       0
  5 Reallocated_Sector_Ct   0x0000   100   100   000    Old_age   Offline      -       0
  9 Power_On_Hours          0x0000   100   100   000    Old_age   Offline      -       790
 12 Power_Cycle_Count       0x0000   100   100   000    Old_age   Offline      -       1343
160 Uncorrectable_Error_Cnt 0x0000   100   100   000    Old_age   Offline      -       0
161 Valid_Spare_Block_Cnt   0x0000   100   100   000    Old_age   Offline      -       42
163 Initial_Bad_Block_Count 0x0000   100   100   000    Old_age   Offline      -       4
164 Total_Erase_Count       0x0000   100   100   000    Old_age   Offline      -       51303
165 Max_Erase_Count         0x0000   100   100   000    Old_age   Offline      -       134
166 Min_Erase_Count         0x0000   100   100   000    Old_age   Offline      -       12
167 Average_Erase_Count     0x0000   100   100   000    Old_age   Offline      -       50
168 Max_Erase_Count_of_Spec 0x0000   100   100   000    Old_age   Offline      -       3000
169 Remaining_Lifetime_Perc 0x0000   100   100   000    Old_age   Offline      -       100
177 Wear_Leveling_Count     0x0000   100   100   050    Old_age   Offline      -       15
181 Program_Fail_Cnt_Total  0x0000   100   100   000    Old_age   Offline      -       0
182 Erase_Fail_Count_Total  0x0000   100   100   000    Old_age   Offline      -       0
192 Power-Off_Retract_Count 0x0000   100   100   000    Old_age   Offline      -       335
194 Temperature_Celsius     0x0000   100   100   000    Old_age   Offline      -       29
195 Hardware_ECC_Recovered  0x0000   100   100   000    Old_age   Offline      -       120445
196 Reallocated_Event_Count 0x0000   100   100   016    Old_age   Offline      -       0
197 Current_Pending_Sector  0x0000   100   100   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x0000   100   100   050    Old_age   Offline      -       66
232 Available_Reservd_Space 0x0000   100   100   000    Old_age   Offline      -       100
241 Host_Writes_32MiB       0x0000   100   100   000    Old_age   Offline      -       430139
242 Host_Reads_32MiB        0x0000   100   100   000    Old_age   Offline      -       450348
245 TLC_Writes_32MiB        0x0000   100   100   000    Old_age   Offline      -       820848
...


The IDs may differ from vendor to vendor and disk model to disk model.

Obviously this is an older disk because of the Old_age tag in the type column.

The best indicator is probably the Host_Writes_32MiB count which is (430139 * 32 MiB) ~14.4 TB. This is almost the 30-fold of the disk capacity.

The Bytes written may have another ATTRIBUTE_NAME on your disk: other commonly used names are Host_Writes_MiB (multiplier: 1.04858 MB) or Total_LBAs_Written (multiplier: 512 B or 4096 B).



If the technician or yourself restored a backup of the old disk to the new disk or installed a new system you can expect the write count to be the "Used" disk space (Disk Utility or Finder Info) and depending on usage about 1-15 GB per day.