Let's say you have a certificate file, foo.cer. The file command can be used to determine if the file is cleartext or binary. In this example, foo.cer is a cleartext file.

~]# file foo.cer
foo.cer: PEM certificate

In this scenario, the cat command (on Linux) can be used to view the content of foo.cer. Something like this should be returned.

~]# cat foo.cer
-----BEGIN CERTIFICATE-----
MIIGhTCCBW2gAwIBAgITFwAS0Zj4+uylATknJgAAABLRmDANBgkqhkiG9w0BAQsF
ADBMMRQwEgYKCZImiZPyLGQBGRYEY29ycDEYMBYGCgmSJomT8ixkARkWCHRocml2
ysR4VfQLr+A3zbM59CQjewP40y7oFgrpNuj8Hp1AXud3nsakEYFaGcc=
-----END CERTIFICATE-----

Let's say foo.cer is a binary file.

~]# file foo.file
foo.file: data

In this scenario, the cat command cannot be used. Attempting to view the content of foo.file will probably return mumbo jumbo, something like this.

~]# cat foo.file
h++-://c_+.+h_i+e++.c-+/ce_+e+_-+

The openssl command with the X509, -text, and -enddate options can be used to determine when a certificate will expire.

openssl x509 -in example.crt -text -enddate

If the file is a binary file, you will also need to include the -inform der option.

openssl x509 -in example.crt -inform der -text -enddate

Which should return something like this.

notAfter=Jul 16 10:13:34 2017 GMT