Using the cat command to view a certificate file (such as example.crt) will display something like this.

-----BEGIN CERTIFICATE-----
MIIGhTCCBW2gAwIBAgITFwAS0Zj4+uylATknJgAAABLRmDANBgkqhkiG9w0BAQsF
ADBMMRQwEgYKCZImiZPyLGQBGRYEY29ycDEYMBYGCgmSJomT8ixkARkWCHRocml2
ZW50MRowGAYDVQQDExFUSFJJVkVOVElTU1VJTkdDQTAeFw0yMDA0MjcxMjU3NDla
Fw0yMjA0MjcxMjU3NDlaMH4xCzAJBgNVBAYTAlVTMQswCQYDVQQIEwJXSTERMA8G
A1UEBxMIQXBwbGV0b24xGzAZBgNVBAoTElRocml2ZW50IEZpbmFuY2lhbDEfMB0G
A1UECxMWSW5mb3JtYXRpb24gVGVjaG5vbG9neTERMA8GA1UEAxMITVFTSVNWQTEw
ggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIBAQDVKIMwyutbQh1J8etzyuzM
bBP1U3JzlQpFjCC+1PvGyeICBb+effaW7Q9kIilXaIeIo0CvGEliQPm4/LHsn5fB
KGKM9zsTSwwEdhOvbTMnCDy9zOKpyahxhYLkOBcbL89CMPt4ShPIY8wMvGa2HjPp
SC6GmCRh0EqeJW9UntGyGoPzLKTBPHdFL2zBr+E1lxVRLb2PUkuNKkhHZZDuaiei
rpZjpfb2Yof393Zfo+oK202DmTvyRiJe8jJAB9KETJGhQHwc6GT66bBitYSrdmyM
A9kNJh4jvsAzEpd5bhS4EQftnpXYv29rjGw1yEsS+32FeA/A0t+c2Sva7g62vdMr
AgMBAAGjggMsMIIDKDATBgNVHREEDDAKgghNUVNJU1ZBMTAdBgNVHQ4EFgQUlBYV
TS88c55UFy7i3pGB7mnGEOYwHwYDVR0jBBgwFoAUc7ve9VRJvf+M2b7HBJTLF7Ve
hLkwggERBgNVHR8EggEIMIIBBDCCAQCggf2ggfqGOGh0dHA6Ly9jcmwudGhyaXZl
bnQuY29tL2NlcnRlbnJvbGwvVEhSSVZFTlRJU1NVSU5HQ0EuY3JshoG9bGRhcDov
Ly9DTj1USFJJVkVOVElTU1VJTkdDQSxDTj1QV1R2VkNBMDExLENOPUNEUCxDTj1Q
dWJsaWMlMjBLZXklMjBTZXJ2aWNlcyxDTj1TZXJ2aWNlcyxDTj1Db25maWd1cmF0
aW9uLERDPXRocml2ZW50LERDPWNvcnA/Y2VydGlmaWNhdGVSZXZvY2F0aW9uTGlz
dD9iYXNlP29iamVjdENsYXNzPBNSTERpc3RyaWJ1dGlvblBvaW50MIIBJgYIKwYB
BQUHAQEEggEYMIIBFDBdBggrBgEFBQcwAoZRaHR0cDovL2NybC50aHJpdmVudC5j
b20vY2VydGVucm9sbC9QV1RIVkNBMDExLnRocml2ZW50LmNvcnBfVEhSSVZFTlRJ
U1NVSU5HQ0EuY3J0MIGyBggrBgEFBQcwAoaBpWxkYXA6Ly8vQ049VEhSSVZFTlRJ
U1NVSU5HQ0EsQ049QUlBLENOPVB1YmxpYyUyMEtleSUyMFNlcnZpY2VzLENOPVNl
cnZpY2VzLENOPUNvbmZpZ3VyYXRpb24sREM9dGhyaXZlbnQsREM9Y29ycD9jQUNl
cnRpZmljYXRlP2Jhc2U/b2JqZWN0Q2xhc3M9Y2VydGlmaWNhdGlvbkx1dGhvcml0
eTALBgNVHQ8EBAMCBaAwPQYJKwYBBAGCNxUHBDAwLgYmKwYBBAGCNxUIhMq+Q4bw
4j2F1Zs4hPWJF4aF1j4BgeKOGYaByVYCAWQCAQswHQYDVR0lBBYwFAYIKwYBBQUH
AwIGCCsGAQUFBwMBMCcGCSsGAQQBgjcVCgQaMBgwCgYIKwYBBQUHAwIwCgYIKwYB
BQUHAwEwDQYJKoZIhvcNAQELBQADggEBAJ4HXHhSu6zZo85DZszCHs+v3kWtsb0r
HkWbSq5IAyWBkx8RGu4LyXD00Z3YjwL8VkY6XpeCPnvX1EyWnhaRu9mAGLeEvz8j
s3g3vfzaMg1BHx/TB1qH+bZW3EbTSKJpUA6JbMRwxL8+X9EqhvieJyeif7Jxyrfk
c1EuBvwKr6mqx8FpS3i+qRBt5dV22iEwMi6qUXhmWRb+Zk5HMumJkeuWxsmEccCv
7dZ4qtGJt8SYLPRAAxMHGnW889BeY8KmdbfeFOyDAmMXEwvB3K3zKQ8aTR781BWv
ysR4VfQLr+A3zbM59CQjewP40y7oFgrpNuj8Hp1AXud3nsakEYFaGcc=
-----END CERTIFICATE-----

The openssl command with the X509, -text, and -enddate options can be used to determine when a certificate will expire. This command can be run on any file that contains a certificate, such as a .pem or .crt file.

openssl x509 -text -enddate -in example.crt

Which should return something like this.

notAfter=Jul 16 10:13:34 2017 GMT