Unfortunately, the sex of monarch larvae can be determined only in dissection - so you probably don't want to know that badly
However, for your information, male monarch caterpillars will have a testis located in the 6th abdominal segment, dorsal to the gut. If you have a last instar male caterpillar, the testis will appear as two bright red or pink sacs (though often they appear to be one sac).
Sexing monarch pupae is another matter and is pretty easy with a keen eye or a magnifying glass. Surrounding the cremaster (the structure from which the pupa hangs) are a series of rings, called abdominal sternites. Within the first ring (9th abdominal sternite) are several paired black dots next to the cremaster; turn the pupa so that you are looking at these dots. If the monarch is a female, the ring adjacent to the 9th sternite will have a line dissecting it; this line (indicated by the arrow on the diagram below) will be centered between the pairs of dots. Male monarch pupae do not have this line.
Diagram taken from Hughes, P. R., C. D. Radke, and J. A. A. Renwick. 1993. A Simple, Low-Input Method for Continuous Laboratory Rearing of the Monarch Butterfly (Lepidoptera: Danaidae) for Research. American Entomologist 39: 109-111.