I have never cleaned or had cleaned a sensor. My old canon 450d was used for 3 years never cleaned. Yes i do change lenses.
Whats the chances its dirty?
Is it safe to really clean it yourself? and whats the best way?
Possible, but maybe not. Mount a lens longer than 50mm, stop it all the way down (f/22 or lower), set to manual focus and defocus the lens, and shoot a bright, featureless subject (a clear blue sky, clean wall or sheet of paper, etc.). If you see spots, that's sensor dust.
It is safe to clean it yourself, have a fully charged battery, activate the manual sensor cleaning mode in settings, remove lens with camera pointing down. First thing to try is blowing it off with a rocket blower (not
canned air!!). If that doesn't work, use a commercial product. Personally, I use a Visible Dust Arctic Butterfly 724 Super Bright.
Your 450D has a built-in sensor cleaning system. So do our 5DII's, and my 7D does, too. I've had the 7D longer, and used it in worse conditions, and it has no dust. OTOH, I've cleaned my 5DII several times, and it needs it again. So, even if your 450D is clean, the FF sensor seems to attract more dust (Canon thinks so, too, which is why the 1D X is getting an updated self-cleaning system for the sensor).