If you're not sure, it's worth taking time to learn how to grill chicken safely - probably more people are made ill grilling chicken incorrectly than any other BBQ method.
It's easy and once you know, and with a little practice you don't even need to think about it.
First off choose which part of the bird you want. The consensus is that chicken legs have the best flavor, followed by wings - personally I prefer the breast meat.
If you want to marinate it, sauce it, or rub something nice on it do so, they will add a brilliant layer of flavor to a potentially bland meat.
A chicken breast takes 15 minutes or so to cook properly (20 absolute max) - so make sure each facet of the meat gets a roughly equal amount of time facing the heat.
Grilling is a direct BBQ method - i.e. the food is directly above the heat source, rather than offset from it (or in a separate compartment) as with indirect methods e.g. smoking - so the heat is greater.
There is a limit to the amount of heat the surface of the meat will take before it starts to burn, leaving the interior raw. This is where the problems arise - there's an element of balance between heat and cooking time.
Above, the meat is just on the grill, the chicken has a pink, almost wet appearance at the start of grilling.
So, keep it moving and turn it every few minutes, this allows the heat to dissipate into the meat and the surface doesn't over cook.
If you have chicken legs, if you pull on one of the leg joints the it should pop apart with little effort. The procedure is the same for breast, thighs or chicken quarters or halves. Just allow a few more minutes each side for bigger pieces like chicken quarters.
As a side note, some recipes for chicken pieces like tikka can be grilled on metal skewers - the heat is conducted via the skewers inside the meat, cooking it more evenly and safely allowing reduced cooking times.
The usual advice is: When the chicken is cooked, any juices from inside the meat should be clear, the meat should be white and have a slightly moist and almost flaky appearance.
You could cut a piece open and see what color the inside is.
While that is fine (and I've repeated it above), there is a risk of getting it wrong and poisoning your guests, or overcooking "to be on the safe side" and turning you grilled chicken into something like rubber.
The old favorite of cutting into a piece. Raw chicken has a pink translucent look to it - cooked chicken meat is white and slightly flaky, the juice will be clear. Though reasonable advice, isn't very precise - and the juices will leak out.
Best to be a bit scientific about it......
So if you really want to be sure go to Amazon.com and consider buying a digital meat thermometer - the internal temperature should hit 180F or 85C.
Basically all you need to do is push the probe into the thickest part of the meat (as this part will cook last); keep try this until the temperature reaches the required point.
Actually since buying and trying several meat thermometers over the years, I pretty much always use one when I grill or barbecue any form of poultry: here is a review of 3 types digital thermometer.
See? There's no doubt about it - it's not cooked yet.
Last tip; when it is cooked, let the meat stand and rest for 5 minutes or so, the same as any other meat.