iMessage can be checked online by logging into the Messages application from a Mac or other Apple device. There is no way to check iMessage from a PC running Windows without remotely accessing a Mac. This is because all associated messages are sent and received through Apple servers.
To access iMessage from a Mac, select Spotlight by clicking the magnifying glass icon on the menu bar, type in "iMessage," and press Enter. Sign in with the desired Apple ID and password, or click Add Account in the top right-hand corner if the service is already linked to another user.
To remotely access iMessage through a Mac from a Windows PC, both computers need to have Chrome Remote downloaded, installed and set up with all disclaimers accepted. From the Mac, go to the Remote Desktop extension, and get the access code needed to secure the connection. Go to the PC, and choose to remotely connect to another device through the extension. The Mac needs to be online and free from any other users. Enter the access code, and follow the steps to link the two computers. The Mac should now be operable via the PC, and iMessage can be accessed using the method described above.