Transmission oil level check

I have a 2012 Volvo d13 with ishift. Lately I have been getting '''check transmission at next stop". Specially when I put it in reverse. But it goes away once going in drive. I don't know what it may be and before getting it into the shop I wanted to see if it was something I could diagnose on my own. To check transmission fluid there is a side window and it seems full when checked with engine turned off. Should I be checking with engine off or on? From what I remember transmission fluid should be checked with engine on and engine oil with engine off? Am I right?