No matter, I've found the info I was looking for - here's a list of alternative CAPTCHA methods (list found here)
Timed Load: This method simply keeps track of the load time of the form, then compares this to the submit time. If the form was loaded and submitted in less than say 1 second, then you can be pretty sure the form was submitted by a bot, and the data should be discarded.
Hidden Fields: Sometimes known as ‘Honeypot Traps’, these are empty hidden fields which can be added to a form and checked for filled values on processing. This can be done using the hidden input type in HTML, or by using CSS to hide the input from the user (but not from bots).
Tokens: Generating a one-time token for a user on form load, and comparing this when processing the form. This blocks externally made requests to the form, (a common method for bots and malware).
Submission Confirmation Page: This method involves adding an extra page between the form and a submission, where users are asked to confirm the details they have entered. This method works because most bots are designed for single page entry and submission of form data. Implementing an extra page in this manner for every form does force extra interaction from the user, but has the added benefit of giving them an opportunity to verify the information entered, which can lead to more effective data capture.
Image CAPTCHA: Use easily recognisable images and have directions such as ‘Click on the Monkey’. To an extent this solution is moving the problem (as we are still asking the user to commit an action to prove they are not a bot) but it can reduce the affect on usability if done well.
Math CAPTCHA: Ask the user to answer randomly generated (but simple) maths questions In most cases this will be simpler than standard CAPTCHA, however this still requires the extra user interaction, and some bot makers are becoming wise to this method due to it’s popularity.
Variable CAPTCHA: Use a standard CAPTCHA solution but only use this when activity is suspicious (multiple submissions from the same IP address for example).
Log everything / Discovery: This is not an alternative as such but a way for you to possibly discover patterns with bot form submissions which could allow for simple filtering rules for form submission data. (‘Log everything’ is always a good mantra when working on the web).