Use boolean variable and check if the file exists
PHP Code:
new bFilePrecached = false;
public plugin_precache()
{
if(file_exists("models/player/blue/blue.mdl"))
{
precache_model("models/player/blue/blue.mdl");
bFilePrecached = true;
}
}
PlayerSpawned(id)
{
if(bFilePrecached) cs_set_player_model(id, "blue");