If I fit a time series with a AR model, I can tell if this fit is valid by checking the residual correlation (valid if not correlated). 

But if I fit it with a MA model, how do I tell if this fit is valid or not? In R, when I specify to fit the time series with, say MA(1), it will automatically use white noise for epsilon, right? 

$y\_t = \epsilon\_t + \text{coefficient} \cdot \epsilon\_{t-1}$

Thanks