Alrite "Stealing Debris" is not the same as "fleet crashing" .
First there's fleet crashing , the term stands for itself so you destroys somebody's flete, and by doing so you make a big debris field around the planet you attacked , and the debris field is created when any amount of any ships is destroyed , the debris just floats around the planets untill somebody picks it up .Now if you were purposly going for destroying somebodys flete you would send in advance your recyclers which are the only ships that pick up debris (just when sending a recycler in right before you send it instead of normaly to send to planet you pick "DF" which stands for debris field and then after you click continiue the only option will be harvest ), so after you send your recyclers in advance then you send you attack and that way nobody but you knows that there's going to be a debris at that and that point, that way nobody can't steal it.
Stealing debris is when for instance you are browsing the galaxy and you find a debris field of 400000 metal and 200000 crystal and you send your recyclers and pick it up then you stole it because it was somebody elses , well not tehnicly , but somebody had his recyclers going for it for sure ,
Flete crashing is a much better way of geting resources from somebody than raiding the planet which has more resources, because the biger the flete is the more debris field there will be so if the player has 40 battleships and you attack him with 140 then you will loose nothing and the player will loose all 40 of them and those 40 battleships will go into debris creationg enaugh debris fro aprox 38 recyclers, and one recycler caryes 20000 resources. Go figure.
To the last question if the player is flete saving the only way to know is he flete saving is if you have a sensor phalang wich is a building that can only be built on moons so you need to have the moon to be able to use the phalang , and the phalang is the device wich scans the players flete movement so you can see if any of his flete is atacking somebody or if he's transporting something or anything , and if you have that information and have flete strong enaugh to take his out you can calculate how to intercept his flete and send your attack so it arives secundes after his flete comes back from whatever it was doing and in that way the player won't be able to do nothing but please you to call back your attack. so hope it helps ,
Well again like i said hiting an undefended planet is easyer couse you need only transporters to stel his/hers resources but later on in the game everybody is going to have defence so you will need to send alot of ships just to go trough somebodys defence , which wont pay of but if that person had some defence with flete then if your flete is stronger you attack and calculate using either speedsim or dragosim how much debris there will be and send recyclers as well.
There you go . 8)