Depending on the type of gearbox you have in your car, you should be able to find a fill plug on the side somewhere. Mine is a type N, 5-speed box and has the plug on the passenger side. It requires a hexagon type socket which you will have to buy I'm afraid. The gearbox takes 3.3 pints of oil,
There is no oil dip-stick on the Capri gear box. You should undo the plug and get the car as level as possible, and then poke the reverse end of an Allen key in the hole: if it makes contact with oil, then everything is OK. If not, then top up with SAE80EP* gear box oil until the oil drips out of the plug hole. That's really all you can do; the oil in the gearbox should last the life of the box unless you have major problems. My Capri takes 3.3 pints.
* Footnote: modern opinion is to use ATF fluid (automatic transmission fluid) type Dexron 2 in the gearbox so it might be worth finding out what exactly is in your gearbox before you start. This agrees with the information given in Chapter 13 (supplement) of the Haynes manual, and improves cold gear changing. My Capri now has Dexron 2 in the gearbox.