Ya sound it out & hope for the best, if you can't look it up/don't have a dictionary. I had this issue the other night & for the most part, sounding it out works fine. Sure, I went with 'tonusama' instead of 'tonosama' that way, but they knew what I was talking about.
If they know what you're talking about, you're fine. People who judge super harshly on needing perfect spelling are either school teachers, or people I don't want to hang around with, since they're being petty about spelling even though they obviously knew what I was talking about. Shrug.
That said, I did get better at spelling by not relying on looking it up. I do all of my writing in a notepad & don't use spell-check at FIRST. I go back later. In general this has made me somehow better remember how to spell stuff right the first time, after repeated mistakes, of course. Obviously not a technique for everyone, but it helped me with college.
I don't really look up the spelling of a word too often. I usually just look up definitions of words I'm uncertain of. I guess kibosh is that one word I looked up recently because I forgot what exactly that cool sounding word I read a few weeks ago was.