I don't know anything about phones.
how do you know if a phone is locked?
can they be unlocked?
Frosty.
If you bought the phone with/on a contract from a cell company, the chances are it will be locked to their system. Locking is usually done by adding a small bit of code to the phone's operating system, which then looks for the cell company's sim to be installed before the phone o/s will run.
If you bought the phone independently from a shop, then added a Sprint sim card later, then it is possible that your Samsung is unlocked.
Only way to tell definitely, is to get/borrow another sim card, pop it in and "suck it and see".

Some cell companies, not all, can make it quite hard to unlock their product. If you don't feel comfortable with doing it yourself as "medic3500' suggests, as it can be quite easy with some to "mis-step" the procedure and brick the phone, then bring it with you.
Most cellphone repair shops here will do it for you as you wait (5 or 10 minutes including merienda LOL!) for a small charge.
I would suggest, if you do bring it, bring the Sprint sim card with you as well. I had one locked phone which needed the original supplier's sim in it to start the unlock procedure.

For the long term, I would suggest when you get a sim card here, it is on the same network as your nearest and dearest - if any- as the inter-network charges are extortionate.
My wife has a dual sim phone (one model of the J# is dual sim, is yours?) which she fitted both Smart and Globe pre-paid sims. This way she minimises the inter-network charges, but can still talk to her friends using the sim of the company they are with.
Both Smart and Globe sell their pre-paid sim cards for around 30 or 40 Pesos, so it doesn't break the bank

I don't have any info on their post paid plans, but they normally want you to have some form of "residence status" before offering a contract.
Hope this helps.
Peter