Are my group messages being sent as MMS instead of SMS?

Hi there! I just have a quick question about text messaging on my iPhone. I read here (http://support.apple.com/kb/HT5760) that "some carries automatically convert group messages into MMS" and that "group messaging is sent either as an SMS or MMS"

See, I am a new postpaid subscriber. And my plan only includes unlimited text messaging (SMS) and data use. I usually send out group messages to 50+ of my colleagues when I need to remind them about something or ask a question. Are these messages being sent as an MMS, or as regular text messages? I fear I may be getting charged hefty sums for texting.

Thanks!