If you don't happen to have the RFM on your lap when you start your approach you can remember what I was taught many years ago in basic training. If you have a ROD of more than 400 fpm and your IAS is less than 40 kts then you should be aware. 400 fpm and 40 kts. Easy to remember, will keep you safe and has worked for me in all types that I have flown.