The specifics of what an investor might try to look for may vary depending on whether the startup is, say, a daily deals play, a chip company, a mobile app, or a genomics startup.
In general, here are the key things being examined with respect to a technical co-founder. Different investors might give differing weightages to these factors:
(1) Technical vision for the product and their ability to actually deliver on it.
(2) The unique technical/scientific insight they bring to the product and/or what are the confluence of technical factors/trends/improvements they are capitalizing upon.
(3) What have they done prior to the startup that can provide some degree of confidence for item #1? (e.g., companies they have worked at, schools they have graduated or dropped out of, etc.)
(4) What are the dynamics between the technical and non-technical cofounders? Have they successfully worked together before in another intense setting? If not, are there signs that can work together now?
(5) Will the technical co-founder be able to attract, scale up, and manage the tech team? (Many VC firms are increasingly helping out their portfolio companies in attracting top tech talent these days.)
If there is interest in continuing discussions with the co-founders, a VC firm will often have CTOs from their existing or past portfolio companies have detailed discussions with the technical co-founders to get deeper insights especially on items #1 and #2 mentioned above.
The majority of investor perception of pre Series A technical founders can be broken down into two categories. Evaluation by product investors and evaluation by non-product investors.
If the investor is a product person themselves then it boils down to their evaluation of what you've already built. Not from a truly technical evaluation (no one in early stage truly looks below the hood) but just "does this person have good product sense and is the vision beyond what they've already built compelling?" If so, that's 75% of the evaluation about you. The other 25% is any evidence about how well you work with others and whether you're aware of your blind spots.
But if the investor is not a product person themselves, then they're less able to assess you on your true merits so they look for:
Pedigree: If they graduated university, was it from what they consider to be a prestigious university? (As a high-school drop-out myself, I don't care about pedigree but the vast majority do especially absent product validation).
Notable Jobs / Projects: What have you done that has worked before and what was your role in it's success?
But ultimately, your own assets or deficiencies are accentuated or detracted by the rest of the team (if there is one), and evidence that what you are pursuing is worthwhile and working (or lack thereof).
Happy to talk with you further if I can be of any help.