If the website is fairly old you could also use Google Webmaster Tools: https://search.google.com/ to do this type of research. Go to Performance > click +New tab and enter the page URL. You should check if ppl are searching for singular or plural more.
Now the importance of this is based on user intend, you should ask your self the following question: "Do you provide a LIST /MULTIPLE or things / services or one single service?"
Google may show your page higher in rankings if you use plural and you provide multiple services as it's more relevant. Once you start ranking for plural and users interaction is good (ppl stay on your site, they click action button etc) you will also rank for the singular.
Depending on what you provide you should write the title according to this principle: If you only offer one single thing write singular, otherwise always use plural.
Check out the competitors, check out their title, do they use singular / plural or both? How long is their title, how often they use singular how often they the plural in title.
Another trick: Let's say most searches go for "best service" as singular but you notice a lot of queries with "top","list" or other combinations. As a test you may want to use both plural ans singular in a single title combining this with other relevant searched keywords. For example I would use: Selling Best Services - Top Service List for the situation above and see what happens in a few weeks.
