Anyone outside of your network, including your ISP, will only be aware of your external, routable IP address. What will be visible to the world are the ports accessible on that IP that are being redirected to the RFC 1918 addresses on your local network.
The only way to conceal those is to lock them down when you don't need to allow a connection through them, or to reassign them to non-standard ports, as most ISP's are only bothered about ports 25 and 80.
So far, ISP's have not done any major crackdown on customers merely for having those ports open. They monitor traffic and check on ones generating a lot of throughput on the assumption they are hosting porn, warez or a commercial site.
There is software which uses TCP sequence numbers to detect NAT. Various forms of through-nat fingerprinting can also be used to make a stab as to wether there is 1 or >1 machines behind a router.
The cost and effort for an ISP to detect NAT is cost prohibitive when the only result is to terminate the customers account. ISP's have more pressing security and usage abuse matters to invest money in than to look for home users who use NAT.
Home Lan environments using Nat are very common, so feel free to join the rest of us who are doing it now.