It will probably be internal but whenever I've had one, it's always been a very thin wand and once you are over the unusual situation, you'll be fine in terms of comfort.
I believe 6 weeks is the VERY earliest a hb can be detected so if you are even a day out with your dates (not necessarily with lmp but if fertilisation happened towards the end of the life cycle of the egg), you may not see it. Try not to be alarmed (easier said than done, I know). If they can't see a hb tomorrow but the sack is the right size, they will probably ask you back in a week. I had this with pregnancy with dd so this time, I've waited until I will 7weeks 3 days to have first scan, just to be sure.
It works the other way too - you could have dates out in your favour and see a heartbeat!
This website is really useful in terms of preparing you for what you will see at scans:
https://www.baby2see.com/development/ultrasound_sonogram/first_trimester_scans.html#week13
Good luck... It will be magical seeing your baby for the first time!x