Good application for detecting state (health) of SSD disk? On my MacBook, I want to see the health status of my SSD. How can I do this? Which app to use?
Command Line

Use built in diskutil to get the SMART status of your drive.  Anything other than "Verified" and you have issues 

diskutil info disk0 | grep -i SMART


Smartmontools

A good open source command line utility is smartmontools.  This provides more detail reporting of the SMART attributes.

$ /usr/local/sbin/smartctl --attributes disk0
smartctl 6.6 2017-10-29 r4579 [Darwin 15.6.0 x86_64] (daily-20171029)
Copyright (C) 2002-17, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF READ SMART DATA SECTION ===
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
  5 Reallocated_Sector_Ct   0x0033   099   099   010    Pre-fail  Always       -       5
  9 Power_On_Hours          0x0032   095   095   000    Old_age   Always       -       21858
 12 Power_Cycle_Count       0x0032   099   099   000    Old_age   Always       -       48
177 Wear_Leveling_Count     0x0013   096   096   000    Pre-fail  Always       -       66
179 Used_Rsvd_Blk_Cnt_Tot   0x0013   099   099   010    Pre-fail  Always       -       5
181 Program_Fail_Cnt_Total  0x0032   100   100   010    Old_age   Always       -       0
182 Erase_Fail_Count_Total  0x0032   099   099   010    Old_age   Always       -       5
183 Runtime_Bad_Block       0x0013   099   099   010    Pre-fail  Always       -       5
187 Uncorrectable_Error_Cnt 0x0032   100   100   000    Old_age   Always       -       0
190 Airflow_Temperature_Cel 0x0032   065   044   000    Old_age   Always       -       35
195 ECC_Error_Rate          0x001a   200   200   000    Old_age   Always       -       0
199 CRC_Error_Count         0x003e   100   100   000    Old_age   Always       -       0
235 POR_Recovery_Count      0x0012   099   099   000    Old_age   Always       -       21
241 Total_LBAs_Written      0x0032   099   099   000    Old_age   Always       -       50124018873


Use smartctl -h or man smartctl for more complete descriptions of command line arguments and examples.

Disk Utility

Using the built in app Disk Utility (Applications>>Utilities), it will tell you give you the SMART status.  Again, "Verified" means it has passed with no errors.



DiskDrill (Free)

The free version of DiskDrill recovery software will provide you with detailed SMART reporting and by clicking on any of the status links ("Healthy" in the image), you get detailed SMART info