Compare WAN IP addresses on your Mikrotik to IP address shown by some internet-based service (e.g.
https://www.whatismyip.com/ ) ... obviously you'll have to do some tricks get results for both WAN links. If IP addresses are the same, then ISP doesn't do additional NAT (but may still run firewall so your connection doesn't have to be completely transparent and open). If addresses are not the same, then you're behind ISP NAT.
Another hint, pointing in direction of NAT, is WAN IP address on Mikrotik. If it's one of:
- 10.0.0.0/8 - 10.0.0.0-10.255.255.255 (RFC 1918)
- 172.16.0.0/12 - 172.16.0.0-172.31.255.255 (RFC 1918)
- 192.168.0.0/16 - 192.168.0.0-192.168.255.255 (RFC 1918)
- 100.64.0.0/10 - 100.64.0.0-100.127.255.255 (RFC 6598 - CGNAT)
then ISP does NAT for sure.
[edit] fixed 192.168.0.0/16 network address