Unless a property is new, or you have it newly rewired yourself, it is hard to know how old the wiring is and, crucially, if it is safe.
There are tell-tale signs of old electrical work, such as surface-mounted cables, old-fashioned light switches, flexes and ceiling roses, and fuse boxes that use fuse wire, rather than pop-out fuses. However, fuse boxes are sometimes replaced without the rest of the wiring being updated, and many with pop-out fuses don’t comply with current regulations.
The only way to know for sure is for a qualified electrician to check the electrics. This should be done every 10 years anyway, even if you think everything is fine, and whenever you buy a property, as if you do need to rewire, it can cost thousands of pounds.
A full rewire isn’t always necessary - your home’s wiring may just need updating, such as adding or moving sockets or wiring in new light fittings. Smaller jobs like these are less expensive and disruptive than a full rewire, but they may reveal problems with the wiring that you hadn’t anticipated.
If you are doing major building work, the electrics in the whole house or flat will usually need to be tested in order to get a completion certificate for the building work.
It’s legal to do some, minor electrical work yourself, but if you don’t know what you’re doing or just want to play safe, employ a qualified electrician (checking their credentials and references). The easiest option is to use an electrician who belongs to an approved scheme such as NICEIC (see www.niceic.com).