Gollgagh said: How in the heck are you supposed to differentiate カ and 力?
edit: derp, the furigana.
But still, without those, how would you?
The only other possible real meaning would be if the "ka" was written in hiragana and used to indicate a question. "Hiroinka" just doesn't make any sense, and in this context the sentence structure itself makes "chikara" a much more likely candidate. Having said that, it was a good touch by artist to add the furigana so as to avoid misreading it altogether.
Generally speaking, it should be quite evident from context.