Copy and Sync use size/modtime by default.
felix@gemini:~$ rclone sync /etc/hosts GD: -vv
2020/01/25 15:19:47 DEBUG : rclone: Version "v1.50.2" starting with parameters ["rclone" "sync" "/etc/hosts" "GD:" "-vv"]
2020/01/25 15:19:47 DEBUG : Using config file from "/opt/rclone/rclone.conf"
2020/01/25 15:19:47 DEBUG : hosts: Size and modification time the same (differ by -519.988µs, within tolerance 1ms)
2020/01/25 15:19:47 DEBUG : hosts: Unchanged skipping
2020/01/25 15:19:47 INFO :
Transferred: 0 / 0 Bytes, -, 0 Bytes/s, ETA -
Errors: 0
Checks: 1 / 1, 100%
Transferred: 0 / 0, -
Elapsed time: 0s
2020/01/25 15:19:47 DEBUG : 5 go routines active
2020/01/25 15:19:47 DEBUG : rclone: Version "v1.50.2" finishing with parameters ["rclone" "sync" "/etc/hosts" "GD:" "-vv"]
felix@gemini:~$ rclone sync /etc/hosts GD: -vv --checksum
2020/01/25 15:19:55 DEBUG : rclone: Version "v1.50.2" starting with parameters ["rclone" "sync" "/etc/hosts" "GD:" "-vv" "--checksum"]
2020/01/25 15:19:55 DEBUG : Using config file from "/opt/rclone/rclone.conf"
2020/01/25 15:19:56 DEBUG : hosts: MD5 = 9da946a118249403281f7ad1e178277c OK
2020/01/25 15:19:56 DEBUG : hosts: Size and MD5 of src and dst objects identical
2020/01/25 15:19:56 DEBUG : hosts: Unchanged skipping
2020/01/25 15:19:56 INFO :
Transferred: 0 / 0 Bytes, -, 0 Bytes/s, ETA -
Errors: 0
Checks: 1 / 1, 100%
Transferred: 0 / 0, -
Elapsed time: 0s
2020/01/25 15:19:56 DEBUG : 5 go routines active
2020/01/25 15:19:56 DEBUG : rclone: Version "v1.50.2" finishing with parameters ["rclone" "sync" "/etc/hosts" "GD:" "-vv" "--checksum"]
You have to turn on checksum.
rclone check specifically validates the size and checksum. You can remove the checksum and just check size only. If the goal is to validate, just re-run the copy or sync and it would check the size/modtime and not copy anything.
If you can see it doing checksums, can you share the -vv output?