check.js

const pem = require('pem');
const fs = require('fs');

let pemFile = fs.readFileSync(process.argv[2], 'utf8');
pemFile = pemFile.replace(/BEGIN PRIVATE KEY/, 'BEGIN RSA PRIVATE KEY');
pemFile = pemFile.replace(/END PRIVATE KEY/, 'END RSA PRIVATE KEY');

pem.getPublicKey(pemFile, function (err, result) {
  if (err) {
    console.error("ERROR", err);
  } else {
    console.log("ok");
  }
});

package.json

{
  "name": "pem-check",
  "version": "1.0.0",
  "license": "MIT",
  "dependencies": {
    "pem": "^1.8.1"
  }
}