To find out if someone is a Snapchat Plus user navigate to their profile, if they have a star badge next to their name that looks like this ✪ then this shows that they have signed up to Snapchat+.
On their profile it will look like this:
Be aware that you can turn the star off in the Snapchat+ settings so not having the star doesn’t necessarily mean they are not subscribed.
If your friend starts asking you why they are only your 4th best friend then this is a good indicator, as one of the Snapchat Plus features allows you to see where you rank in people’s best friends lists.
If they also seem to have eerily good knowledge of exactly where you have been then this may also indicate that they are Snapchat+ users as the ghost trails feature allows them to see where you have been in the last 24 hours, providing you are sharing your map location with them.
Other than that, the best way to find out is to ask them directly!