Jesus h Christ you monitor the profile from the chart , ie at this waypoint , what height am I meant to be.
...and other bothersome things like deviation ....
If you are, then it doesn't get 1000 feet low without you noticing and doing something about it.
Lord give me strength
Until on final, I can't see many charted altitude limits or profile; the RNAV Zs got none apart from the holding pattern limits. Of course, the VNAV deviation thingee (I don't fly bussses) would probably give a clue.
"Use the force, Luke, but just in case, make sure your targeting computer is engaged in VNAV!"