Remember that players do sleep and come from different time zones so they might not be active in the last battle which is why you should always check a few.
Epic Battle History: The best and easiest way to check activity is to look at their clan Epic Battle history and check the last few epic battles to ensure their name is on there and this also lets you see the number of hits they made and how much they earned which is additional useful information.
They must be active because activity means they are generally earning money which means they will grow and in turn means you have a chance of selling them. In fact, it's the most important quality any ally you buy must have.
