Way to check if SAP system is ABAP based, Java or Dual stack

Using SAP GUI, you can only access ABAP or ABAP+Java based system. You can check at multiple places if it is a dual system.

T-code: SMICM, you can checklist of services in transaction SMICM (Goto --> Services) - dual stack systems will have the J2EE services listed too. There will also be an AS Java menu in SMICM on ABAP+Java systems.