If using Samsung Messages (stock app), you can see if it was MMS after it sends because it will say "MMS" under the text, and also if you long-press the text, a menu pops up and you can choose to see the info. on the message (same for received texts).
Also, if you reach the character limit for MMS texts, it will tell you and not let you write any more in the box, and they will send as five text blurbs, the first four of which are the kind you tap to open to read all of.
