I have been estimating for 10 years. I have hired and trained over 100 estimators personally in that time. This puts me in a unique position to assess and share what I think makes a good estimator.

Ability to read and visualize drawings

This is by far the most important skill needed. If you can't understand the drawings clearly you will make mistakes. While a majority of people will develop this skill with practice, some people just find it very difficult. While hiring, be sure to test this by sitting in front of a computer and asking questions to assess plan reading ability. 

Focus

Accuracy is everything in estimating. And to do that you need to focus for long stretches. Often you need to keep 3-4 pieces of information in mind while doing the estimate. Any distraction or break can then take at least 5-10 minutes, just to reorient and remember all that information before beginning again.

If a candidate is constantly checking their phone or their phone keeps beeping with notifications, then that is a big red flag. To be a good estimator you need to keep doing a task for hours at a stretch. Frequent breaks in concentration are bad for both accuracy and productivity.

Basic Math

This is quite self-explanatory. Yes, software and calculators can do most of the work. However, I have found out the hard way that that isn't always enough.

Just as an example, I ask the following question in interviews - How many floors are there from the 11th to 57th floor of a building? You might be surprised at how many get this wrong. Imagine missing a whole floor in your estimate.

English comprehension (even for native speakers)

The kind of notes I have seen in drawings over the years range from the bizarre to the incomprehensible. I didn't search around for the most difficult or bizarre ones. Instead, below is note that I found just by opening a random drawing and reading only 3 notes. It's not very difficult, but again not everyone can comprehend this based on their English proficiency.

Differentiate between what can be learnt and what can't

Estimating often requires construction knowledge specific to your trade or business. This can all be learnt. Don't get hung up on this. Same goes for any estimating software you use.

What can't be learnt (at least not for practical purposes) are the things I mentioned above in the blog. The basic building blocks that help an individual estimate accurately. Even if they can learn on the job, its too much of a risk hiring a candidate who lacks any of them.

Instead, give them proper feedback and where you think they are lacking. This is not just a nice thing to do but will also help you get clarity on the reason behind rejecting a candidate and reduce bias.