This worked for me:
I'm pretty sure that on all BMWs, the shift lever will always move freely up when your in top gear (6th) and freely down in bottom gear (1st) when you put the slightest pressure on the lever (clutch lever out) so can always check when your in 1st and 6th.
As Moybin mentions above, I think you'll find that only in 4th gear the tach and speedo needles move in near perfect parrellel throughout the entire rev range.
Knowing this, if the tach needle is on a steeper angle than the speedo, you're in 3rd. If the speedo needle is on a steeper angle than the tach, you're in 5th.
Assume you can easily guess when you're in 2nd.
Try it... it works at an easy glance and no calculations.
