How to detect our session mode in Windows

Dear colleagues,
I have a BIG headache when I wish to toggle window mode from fullscreen to windowed and vice-versa with DOS applications. I need first to know in which window mode is beeing exhibited in one session. Imeans in WINDOWED mode or FULLSCREEN mode. And I do not know how to do it. Some help in language C will be appreciated. Normally It will be used for MS-DOS proposes.
Sorry, my English terms are probably not so good. But I will try to explain what I looking for.
I need to develop an application, with which I would check first in which screen mode is my session beeing shown (FULLSCREEN or WINDOWED). After getting results, I would like to show a message to the users, in order advising that they need to press ALT ENTER to change screen mode (this in case the screen mode is being shown in the intended mode).
The second step (I means, when I have got screen mode), is to force screen mode, changing from FULLSCREEN to WINDOWED and from WINDOWED to FULLSCREEN.
I will tell you, why I needing this. I work with MS-DOS softwares. But I also need to work with GUI apps. In others words, my system is hybrid (DOS & GUI). Sometimes I need to execute a GUI from my DOS apps, but my DOS apps become minimized window. After this I wish to return to DOS apps in the original screen mode. In DOS apps, user usually works or prefer to works in FULLSCREEN not in WINDOWED, and this makes difficult to restore window after a GUI apps.
It is really difficult to get a solutions, due my C language knowledges. But if I can count with you to clearfy which way I could take, or what to use, I would get a extra help.
I understand is not so easy to detect screen mode. And all your advising will be appreciated and cooperations will be announced like as a new solution of old problem with DOS apps.
 

I will be very gratefully your help.

Thank you for your attention,
B.Rgds