OpenSSL

View details of a certificate (.crt file) :
openssl x509 -in /mypath/ca.crt -noout -text

Same thing but for cert request : openssl req -in CSR.csr -text

Convert a crt to pem :
Officially :
openssl x509 -in mycert.crt -out mycert.pem -outform PEM
In practice the pem has often the same content than the crt.

Verify a certificate (pem format but crt works in most of cases) against a specific root CA :
openssl verify -verbose -CAfile foo-ca.crt foo-app.crt

Variant :
Verify a certificate against an intermediary certificate :
openssl verify -verbose -CAfile foo-ca.crt -untrusted foo-intermediate.crt foo-app.crt