C - Calculations - Are you using the right formula? Are your numbers right? Do you get the same answer if you do it another way (etc. Different rule, or by calculator/hand)? Are there enough to get the full marks for working out? Can you read it? More importantly, is it near enough that someone who is tired and seen it 489 times before going to bother? *
U - Units? A mistake made commonly enough in further (and maths in general?) that it deserves its own letter. Pretty self explanatory, but make sure that your letters at the end don't look like numbers. (etc. 40 km or 401 cm? Maybe it's just me, but handwriting is the real test here)**
T - True? If you substitute your answer back into the equation or whatever, is it true? What if you use another method? Does it match up with the theory? (Sure, 1.1 + 0.9 + 1.75 + 1.25 = 5, but what kind of question on Seasonal Indices uses a five season period when your working out only includes 4?)
E - Explainable? Kind of like the context check, so mostly for worded questions, but does your answer actually answer the question? Sure, Billy is arrives at Narnia when t = 3.5, but the question says he left at 8am (so t=0 at this time) - you probably won't get the marks unless you write 11:30am.
*A note on this, if you don't use custom functions on your CAS, you're missing out. Saves time and silly mistakes that you could make typing it out or doing it by hand. Just make sure it's right before you go into an exam relying on it

**Similar to the other note, I use custom functions for conversions because I'm terrible at them. I also have a table in my summary book for checking by hand. Maybe you don't need it, but I, personally, am terrible but would still like a nice score at the end.