As far as I recall, if we can (that is, if physically possible), it used to be that we'd at least do a deep bow towards the Tabernacle (meaning head, shoulders, waist), but if the Blessed Sacrament was exposed, then we would at least genuflect, and adore with double genuflection (on both knees). From what I read, now:
Since a genuflection is, per se, an act of adoration, the general liturgical norms no longer make any distinction between the mode of adoring Christ reserved in the tabernacle or exposed upon the altar. The simple single genuflection on one knee may be used in all cases.
However, some bishops' conferences have voted to retain the use of the double genuflection for the Blessed Sacrament exposed, and it would be required in these countries. In this case, those who make the double genuflection kneel briefly on both knees and reverently incline the head with hands joined.
Needless to say, the simple genuflection should never be reduced to a sudden spasm in the right knee. The right knee should touch the place where the right foot stood while head and back remain straight. The gesture of adoration should be performed with due pause.
A reader from Kalgoorlie, Australia, asked what is the proper act of reverence for those who are physically unable to make a genuflection.
in this case the general principle "ad impossibilia nemo tenetur" (the impossible obliges nobody) is applied. It is enough for such people to do whatever act they are capable of: a deep bow; slowly going down on the knee in a pew; or, if even this is impossible, fulfilling the essential act of adoration which is an interior and spiritual movement of which the external gesture is an expression.
To willfully choose not to genuflect or even bow towards a Tabernacle or, much worse, to the Blessed Sacrament, is irreverent towards God and it constitutes sin.
The following account of Padre Pio is rather well-known:
One night I was alone in the choir and I saw a friar cleaning the altar late at night. I asked him to go to bed since it was so late. He said: "I'm a friar like you. I did here my novitiate and when assigned to take care of the Altar, and I passed many times in front of the Tabernacle without making the proper reverence. For this sin I am in Purgatory
God is there.
You are not being scrupulous: we will never be able to show proper reverence to God, but at least we should try with all our heart and mind to do so.