Blood sugar levels?

I have a quick (very stupid) question. . .How do they check for sugar levels?? I'm assuming it is a heel stick?  We had a bad experience with the nurses when DD was born and talking to friends, it sounds like it is a common practice at our hospital (we thought it was because she was early and they were worried), which I would like to avoid with this LO.  I know a lot of you had to watch sugar levels and thought you might know.  Thanks!

Back story, if you care: DD was born at 35 weeks.  Because she was regulating her temperature and breathing fine, the nurses insisted she was having trouble eating (even though we were feeding every two to three hours with corresponding wet/poopy diapers every one to two hours). Eventually they told us her blood sugar was dropping since she wasn't eating enough and we had to feed her formula or they would put her in the NICU (she han't been in there up till this point).  They started giving her 2oz of formula after every nursing session, which she screamed and spit up through (the formula, not the nursing). . .and then would promptly fall asleep for four to five hours, even if we tried to wake her to feed her.  Then the nurses got mad that we weren't breast feeding more and got on our case even more and wanted us to stop breast feeding all together.  They were also mad that she took 45min to an hour to nurse rather than 15min on each side. 

In addition to being totally stressed and unsure of what we were doing, we ended up having some breast feeding issues, which I think were related to the whole situation. . .But through all of this, they never did any testing on her other than body temperature; so how did they know her blood sugar was low?  I'm all for formula if it is needed, but I think the nurses put us in a very unncessary position so they could fill in their chart (like I said, several full-term friends went through similar situations at the same hospital) and I'd like to know what tests I can ask for before they start encouraging formula.