Well, it was a BW mummy that suggested it. Everything looked like OT but none of the methods to overcome OT were helping (ie shorter A, EBT, I was trying to W2S naps to extend etc). Generally an UT nap would be a happy wake up, chatty, playful, but mine was screaming blue murder. It was because his A was too short for him but he was still going down for the nap, then waking early (not tired enough to transition even with help) but because it was a short nap he was still tired. Really there isn't a clear way of knowing other than if shorter As and EBT don't help then try the other way! I actually prefer an OT nap because I found it easier to get him back to sleep so ultimately he gets more sleep than a short UT nap when he won't go back to sleep.
Your current A is within the guidance times for age although as bec said might be short (depends on high or low sleep needs too), at 10 months some are on 4hr or more A time (possibly not all As being equal length depending what suits).
What you said in your original post though about more activity, that does make them more tired depends how much activity and stimulation she can take.
Sorry - it isn't a clear cut answer is it. If you try ebt a couple of days and don't get anywhere then try a longer A time, increase by 15 mins, hold a couple of days then increase again if needed. Watch her mood prior to the nap too, she might be happy with the extra play time.
