Yes, if you see graft scar on that particular branch and the leaves and the fruits don't look alike as you stated.

If you have a lime tree, snap off a fairly new leaf and smell the newly exposed bark. My lime trees smell very strongly whereas I could not tell a lemon tree by leaf smell alone. Key limes are very small, round and bear in clusters. The lemons I have (Lisbon and Eureka) have very pointed ends.

You can also tell by the leaves. Key limes have very small leaves -- maybe up to 1.5" across by 2" long max (usually smaller) with little or no petiole wing. 'Tahiti' limes have "average" leaves, maybe 3" long, with variable petiole wings, but at least some leaves should have a significant wing. Lemons have among the largest of all citrus leaves -- 4-5" long, with no petiole wing at all. Key limes also make little, "twiggy" new growth, whereas the other two put out much larger flushes, often several feet long.

Lemons can easily be identified when in bloom by the purple blush on the flowers where as other citrus flowers are pure white. If you can't determine which is which then when the fruit are ripe you'll know then. Key limes will stay small and almost round like a ping ball. true lemons get much larger and wil usually have a pronounced nipple on one end even when green.