For a pure AR(p) model, the ACF decays slowly to zero, and the PACF become zeros for p+1,p+2.........
For a pure MA(q) model, the ACF become zeros for q+1,q+2,......, and the PACF decays slowly to zero.
So you can just take a look at the correlogram of the original series, and decide whether it is a Pure AR of Pure MA.
But this only applies to PURE MA or AR processes. Most likely, you will have a ARMA process, then it doesn′t work.
When your model is good enough, your residuals will be close to white noise, so you can validate your model by looking at the residuals, for example:
1 Q statistics (eviews)  
2 Correlogram: ACF and PACF of the residuals ( should not be significant for the first 10 to 20 periods)
3 DW statistics 
or you can use AIC and BIC.
But first of all, make sure the series is stationary, this is very important.
Good luck!!!