The best method is to go to the site and see what players are playing. Nothing beats timely data, and even if the players you observe are not the opponents you will face it indicates where other players heads are. I went to the LCQ the day before the RPTQ to see what players were playing and it was a lot of Burn and Jund. I therefore assumed that the local players thought Jund and Burn were the good decks and planned accordingly.

The other method for an invitational tournament is to look at what got them to that tournament. The old expression “Dance with the one that brung ya” tends to hold true to these things. I looked back on metagame data from the qualifying season and saw that Jund, Infect, Tron, Burn, and Bant Eldrazi were popular then, meaning that it was likely many who were qualified would be playing those decks. If they were winning then, it makes sense that many PPTQ winners would have played those decks. I was right in this case, though it is far from foolproof.

This only works for relatively small tournaments. There’s too much variance at a big event. The best bet there is to look at the metagame breakdowns and hope for the best case scenario.