Sometimes, especially during combat, the gamemaster may appear to be wrong. This might be because they've forgotten something – inevitable, given the number of things that need to be juggled in combat! Alternatively, it might be that gamemaster and player remember or interpret a given rule differently. It is always worth voicing the question when this happens, but it is important that disputes do not disrupt or derail play.
Reminders are quick and straightforward, and players should never hesitate to provide them. Gamemasters will take reminders in the spirit in which they're given – you're not nagging, you're helping the GM keep track of the many moving parts within a combat. Clarifications are similarly easy to deal with.
When the matter proves to be one of genuine disagreement, the book, the FASA forums, the official errata and the house rules determine who is right (in ascending order of authority). Ask for resolution to be delayed until the end of the turn, as if the acting character had delayed their initiative, and allow the gamemaster to continue running everyone else's turns while you look up the details. Usually, looking at the precise wording will resolve the issue, but if the interpretation seems unclear, the gamemaster should always err in favor of the players. (If one person at the table interprets the wording one way, and all others interpret it another, it should be deemed sufficiently clear.) In any case where consensus was not achieved, the question should be discussed with the wider community after the game and clarified in the house rules.
Yes, this means that the gamemaster is Not Automatically Right. The gamemaster is a piece of the puzzle that makes up Earthdawn Eastmarches, but is no more important a piece than any given player. This also means you do not need to be an expert on all the rules to try your hand at running a game – the community will support you as you learn!