A dekko is a look or quick glance, most often said as have a dekko. It’s an old Anglo-Indian borrowing from Hindi dekho, spread around by British Army slang and then stubbornly kept in everyday speech. Use it when you want someone to come over and check something out, usually with a bit of nosy curiosity.
"Oi, have a dekko at their front garden, proper show-off stuff. Ours looks like a windswept car park, so shut the gate before the neighbours clock it."