All of the above. It seems to depend a lot on how long the coach has been in town and how well he feels that scouting is going. If the perception is that there's a lot of undiscovered talent and that the team isn't scouting very well, then they'll announce an open trial. Pretty much every time a team opens up shop (in 1996, and again in 1998) there have been open trials. Metros seem to have one every couple years. San Jose has one most every time they have a management change. Other than that, players that are scouted are invited to train with the team and based on the training trial, may or may not be offerred a contract. There's a thread around here about the ACC scouting going on right now. Most of the college final tourneys are pretty well scouted these days, and anyone who gets onto one of the under-X teams seems to be a target for the league. The top scouted players and some others will be invited to the combine before the draft to see how they match up against the other potential draft picks. Sometimes there are a few surprise players there (Haydin Woodworth last year, who came from a 3rd division school). Seems like often enough players who are more established will get an agent. Agents can organize trials and kind of market their players around. A series of successful training trials overseas even without a contract offer might help MLS get interested in a player.