I respectfully disagree with the above answer (although I will admit that the truth behind it can be proven when looking at the lines spoken by Friar Lawrence above). There is a moment in the tomb when I cannot help but be incredibly frustrated at Romeo's lack of observation! In fact, I get frustrated every time I come upon this part, ... whether reading the text, seeing the play, or watching a movie version! My frustration hinges upon this moment:
Oh my love! / my wife! / Death that hath sucked the honey of thy breath, / Hath had no power yet upon thy beauty. / Thou art not conquered. Beauty’s ensign yet / Is crimson in thy lips and in thy cheeks, / And death’s pale flag is not advanced there. / … / Why art thou yet so fair? Shall I believe / That unsubstantial Death is amorous, / Ant that the lean abhorred monster keeps / Thee here in dark to be his paramour? (5.3.91-105)
Quite obviously (and according to the text), Romeo fails to realize the obvious signs of life that are evident in the “dead” Juliet. Namely, she doesn’t look dead!!! Her lips are still red as there is “crimson in thy lips.” Her cheeks are still flushed as there is also crimson “in thy cheeks.” Juliet is not pale at all; therefore, Romeo says that “death’s pale flag is not advanced there.” He actually muses why this could be so by saying, “Why art thou yet so fair?” I adore the way DiCaprio handles this part in the Luhrmann production of Romeo + Juliet. There is a fleeting moment where their eyes meet as the poison touches Romeo’s lips. This, of course, isn’t in the text but adds just a bit of angst when one would think more angst couldn’t be added.
Poor guy, I suppose Romeo is just a little overwhelmed with emotion at this point. A shame he didn’t notice, eh? Then again, who ever heard of The Comedy of Romeo and Juliet?
There was nothing that could have revealed to Romeo that Juliet was in fact not dead and merely in a very deep sleep. If we look carefully at the play, and in particular at Act IV scene 1, we can see that when the Friar describes his plan to Juliet, it is clear that the drug he gives her is so strong that she will appear to all intents and purposes as if she was dead. This drug is so effective that it manages to convince her family, after all that she is dead. Consider how the Friar describes the strength of this drug:
When presently through all thy veins shall run
A cold and drowsy humour; for no pulse
Shall keep his native progress, but surcease;
No warmth, no breath, shall testify thou livest;
the roses in thy lips and cheeks shall fade
To wanny ashes, thy eyes' windows fall
Like death when he shuts up the day of life...
The drug is clearly represented as being so powerful that it manages to make Juliet to all intents and purposes appear dead to any observer. The Capulet family would not have held a funeral service for her before they had summoned a doctor to examine her and pronounce her dead, and therefore there would have been no indication to Romeo to suggest that she was in fact alive.