If your HSI has a glideslope indicator on the left and right of the compass rose then it is even easier (try drawing it out on a quick sketch of an HSI:
Imagine slanting line from one dot low (one dot UP on the display) - starting on the right hand side if right hand hold, left if LH hold. This slanting line goes down to the one dot high (1 dot down on the other side). Then, vertical line down to the 6 o'clock.
This divides the HSI into the three sectors, actually called sectors 1,2, and 3 officially:
Sector 1 : Parallel
sector 2 : Offset / teardrop
Sector 3 : Direct
Look at the HSI HEAD - if in the top half of the HSI (the biggest sector) = sector 3 (direct)
The smallest segment is sector 2(offset) - on the bottom left if RH hold and bottom right if left hand hold. The remaining medium segment is sector 1 (parallel).
Best bit about this is that if your heading changes you can immediately see if your sector entry is changing. Also, Aerad plates show the 70 degree line on the hold - useful.