Open your router's configuration interface. Somewhere you'll see something like 'external' or 'public' IP. If your external IP is in one of the following ranges, you're basically screwed as long as portforwards go:
10.0.0.0/8
172.16.0.0/20
192.168.0.0/16
100.64.0.0/10
any IPv6 address/range - This shouldn't be a problem, but Minecraft's IPv6 support is still rather quirky. You're at your own but we're happy if you share your experience.
You should ask your ISP for a public and IPv4 address (but this may cost you money). NB: your internal IP should and most probably will be in one of the first three ranges, don't mix them up