Not really an easy way, as it depends a lot on whether you’re just using SSiW or supplementing that with other things, e.g. Duolingo or reading books, and listening to Radio Cymru which extend your Welsh.
SSiW learners often find that they’re a lot more confident speaking Welsh than those who have just used traditional classroom methods, but their vocabulary may be more limited. A very basic rule of thumb would be to go for Sylfaen if you’re anywhere beyond Challenge 14 in Level 1, and give Canolradd a try if you’re halfway through Level 2, but it really depends a lot on how much practice you get with your Welsh now.
There has been discussion on this point before, so if you have a look at Latest FREE weekend course information from a couple of years ago, the conversation there might help you.