Are you talking about cruciform DNA? That's the only use of the word cruciform I can think of in chemistry. For complex systems I would say you don't just decide it in a "back of the envelope" manner. I assume you'd have to have some computer to run all the calculations for you.
However, for a linear conjugated pi system, no matter how many double bonds there are, you can in theory draw out the HOMO. Beyond 8 carbons I personally tend to have trouble drawing all of the MO's of a linear conjugated system. But the idea is still the same as if we were analyzing ethylene... draw all of the MO's of the pi system and begin filling them accordingly with the number of pi electrons you have. Eventually you run out of electrons and you find your HOMO.
Cyclic conjugated pi systems tend to be more challenging to draw IMHO. I think if I could draw the MO's of benzene, cyclopentadiene anion, etc... but anything more challenging I'm screwed.