How could I tell if caregiving is putting too much stress on me?

If you don't like it. If you are experience feelings of frustration, if you get angry easily, if you do not see an exit to your situation, if people in your surroundings notice that your level of tolerance has decreased or you shout, scream or treat bad people or animals. If you don't have time for yourself. If you are tired all the time, if you cry a lot. If you get sick, if you lose or gain lbs.These could be signs.
Inner review. Answering this question requires that you review from time to time how you're feeling inside. Are you rested, or exhausted? Willing to help, or resentful? Are you achy and sore, or feeling fresh and ready for physical work? Are you maintaining outside friendships & activities? Depending on your answers you need to make adjustments, and call in additional helpers so that you remain resilient.