1) If Hydras spawn in the Castle and there's somewhere to retreat to, it's a good spawn.
2) If Hydras don't spawn in the Castle, but there's something to compensate with (hiding spots, a nearby secondary resource) – it's a good spawn.
3) If Hydras don't spawn, there are few hiding spots or they are weak, but you can see several promising resource nodes that aren't too heavily guarded – it's a good spawn.
4) If none of the above conditions are met, consider restarting (especially if you haven't taken them yet).
5) Finding Earth with a Town Portal slightly increases the spawn rating.
6) The goal is to have enough power to take down a Tier 3 Utopia on turn 131-132 (for a 4-player map), and preferably on turn 125-126 (for a 5-player map). The rest are Tier 3 and 2 maps, and it's better to restart them.
7) If 50% or more of the generated maps meet the restart criteria, increase your skill. You don't take the Swamp faction to break resource nodes on turn 3, or Utopias on turn 4; that's the pace for Inferno.
Something like that...