I'm going to Spain for a few months this summer and it's possible that I'm going to stay longer (but not much longer) than the 90 days I believe I'm allowed to stay without a visa. Further complicating matters, I'm planning to do a one-way airhitch over there so I won't have a ticket showing my return date or anything like that.

What do I tell them when I arrive and they ask me how long I plan to stay? Lie and say two months? Is the fact that I won't have a return ticket going to be an issue?

When I flew to Spain last February, they only check my passport at Los Angeles Airport, I flew to Paris to made a transfer to Madrid, in Paris airport they looked at my passport opened it and let me pass, once in Barajas airport in Madrid, nobody checked me and suddenly I was on the street.
there is no stamp or anyother thing in my passport that show that I was in Spain, I do not know how can they will know if I decided to stay more than 90 days in Spain.

To avoid problems maybe you go 2 days to Marrocco in Africa and then go back to Spain, that should be count as a new entrance and you will have another 90 days.

Rafael