If argument forms are always valid or invalid,

It's best to learn what the actual piece of writing is first or the base of the argument. Once you know these two then look at the pattern of the text shown. Ask yourself if they are: factual, idealistic, opinion and so on so forth. However; you should also look whether it's biased or not including whether it's in favor of the other also for example: "I think David Cameron has been one of the best ministers we've had in over a century" This would be seen as favorable biased writing if it was reading a report on politics written by a journalist. It's worth having a look at news reports on politics and religion this may help. It is a way of deciding whats best when recognizing if the argument form is invalid or valid.