[douglas]

You can tell something about what you're going to get from ruins in a random stock map by the description of the ruins.

"There are ancient ruins on this planet from a long dead civilization." - 1 random tech

"There are buried alien cities on this planet that have long been deserted." - 2 random techs

"Sensors indicate some kind of advanced technological devices left on this planet." - 3 random techs

"We have detected advanced technology present on this planet." - predetermined unique tech

Which unique tech you get from unique tech ruins is determined at map creation. Random techs are determined at the time of colonization, and are chosen completely randomly from the techs you have all the prereqs for and haven't researched to the maximum level. I'm not sure, but I think every tech area has an equal chance of being picked. If you're playing sequential movement, it's possible to force a ruins to give you a particular tech by saving before colonizing, and reloading every time you don't get what you want. The random number generator is reset to the same spot in the sequence every time you reload, so you'll have to do something like simulating two colony ships running away from each other a different number of times after each reload. This is not recommended for most players if you want a challenging game, but may even the scales if you're consistently losing to the AI.