The only way to know is to look at the location where you put them. If it is the SD card, you can try accessing the card through another device or try connecting the phone to a computer via USB cord and choose to view what is on the card that way.
If they are saved on the card, then you just have to be certain you delete them from the phone's memory only.
Personally I would suggest backing them up to a computer or cloud account first. Make sure they are actually on the computer or in the cloud account, and then delete them off the phone.