It's not a silly question but it's hard to give a simple answer.
Yes, you'd have to look at the whole synastry between the two individuals. On some sites they list or even color code how compatible each sign is with the other signs and fx. my Cap Sun is at 0,04 degrees/Sag-Cap cusp/1,5 hrs. into Capricorn Sun (my chart is on my profile) and when I look at those charts, at least based on my experience, I've been more compatible with the signs - usually Air and Fire signs - that these cook book astrologers think are a good match for Sagittarius. I didn't mean to sound condescending, cook book astrologers was just short hand for the type of sites most of us go to first when we begin to learn about astrology, and there's nothing wrong in that.
OT - I feel like I have to post happy smileys after every paragraph or sentence, just in case my "tone" gets misunderstood by any readers.
All of that said, I do have Mars in Sagittarius. So maybe that simply explains what I've experienced when it comes to compatibility.