IMHO, it's pretty clear a combination of two things. Before the poisoning:
Around her throat was a red gold choker tighter than any maester's chain, ornamented with a single great ruby.
The ruby at Melisandre's throat caught the light as she turned her head, and for an instant it seemed to glow bright as the comet.
And then once the poison is in the cup:
Red silk, red eyes, the ruby red at her throat, red lips curled in a faint smile as she put her hand atop his own, around the cup. Her skin felt hot, feverish. "It is not too late to spill the wine, Maester." "No," he whispered hoarsely. "No."
She drinks, he drinks, and:
"He does have power here, my lord," the woman said. "And fire cleanses." At her throat, the ruby shimmered redly.
So there's 2 things:
First, she knew that he had poisoned the cup of wine, and even gave him a chance to get out of it. She sees things in the fire, I assume this is an example of the author using "show, don't tell"...he shows us her power and foresight without having to spell it out. The second is that that ruby at her throat is definitely being highlighted--we don't know anything about her rings or shoes or hairpins because they're not important, but the ruby is. The ruby has some power strongly associated with light and fire or at least is a tool that she can focus her power in. Since she saw it coming, she was able to be prepared with the right magical tool, or the right spell.
Later in Chapter 9:
Melisandre was robed in all scarlet satin and blood velvet, her eyes as red as the great ruby that glistened at her throat as if it too were afire.
Since fire is the 'source' of her power, that's a bit too strong of a comparison to ignore.
This is made even clearer in ADWD, as it is illustrated in other ways.