Here is how I would try to gauge the type-in figures. I first look at the Alexa traffic rank. Sometimes, parked domains do not show accurate Alexa rankings due to less frequent rank updates. The next step would definitely be to use a backlinks checker. If a domain has no backlinks and yet receives traffic, it could either be pure type-in or bot traffic. To make sure that it really is type in traffic, I use a wordtracker search query to find exact phrase searches a day. For example, if I am analyzing the word "debt consolidation", these would be my findings.
Exact phrase searches for the word "debt consolidation" is around 3864 a day. I would then try searching for a combined phrase match "debtconsolidation". Wordtracker shows that around 20 people/day search for the combined words. Finally, I would run a search with extension and see that around 3-4 people actually type-in debtconsolidaton dot com. This gives me an idea, with some sampling/software error provision, that atleast 4-8 people purely type-in debtconsolidation dot com each day.
Now, in this case, although the actual site debtconsolidation.com has a traffic rank of 740,926, it isn't the natural type-in traffic rank. This domain has around 2610 backlinks which could account for 80% of the actual traffic source. I would estimate this domain gets around 300 visitors a day from backlinks + type-in +any PPC advertising sources. Without the backlinks, this domain wouldn't get more than 300 pure type-in visitors a month. That means its actual pure type in Alexa traffic rank should be around 1,193,700 or somewhere near this figure.
However, this is still a premium domain, with an estimated value near to a million dollars Imo.