You don't need to be a roof installer to worry about your roof to the extent of wondering if you need a new roof. In fact, anyone with a roof over their head in the capacity of a landlord or tenant should know what signs to look out for if their roof is coming of age and requiring a change. If there is a roof out there that lasts forever, I'm yet to see it. Likewise, a house without a roof cannot be said to be complete.
In general, when it comes to roof replacement, you can choose to have a brand new roof or repair the old one depending on the observed amount of damage. Although, depending on your financial situation and how much life your roof has left, it is perhaps always better to go for the former. Furthermore, when it comes to finding out the exact life span of your roof, the obvious choice is to hire a professional roofer or local roof contractor. Unfortunately, doing so carries its own cost and should roof inspection be required periodically, you are not talking about spending hundreds but probably thousands of hard earned cash over the life span of your roof. Thankfully, you can conduct some of the basic roof checks yourself and access any damages. And here's how to do it.
Considering that there is probably nothing like a simple roof check, you should have a schedule for checking your roof for signs of damage, preferably in early Autumn and Spring.
Next, conduct an interior check by going into the loft/attic, obviously with a powerful torch (that won't set your house on fire). If done in daylight, as it should, some of the signs of damage include light shining in through a potential gap, from outside. A basic rule of thumb for most roofers is that "if light does get through your roof, so can moisture". Hence, you shouldn't ignore the situation.
Other signs include sagging due to external pressure or moisture accumulation. This can also come in the form of dark spots or trails. If you find any signs of moisture damage, you can confirm this by looking out for poodles or signs of water leakage.
Once, you have completed the interior check, it's time to conduct an exterior roof check. This typical involves finding a safe and secure way to the top of your roof to look for visible signs of damage. When you do so, you don't want to escalate the situation so avoid walking on your roof as doing so carries a lot of risk for you and your roof.
Some of the most visible signs of exterior roof damage include split or cracked shingles or tiles. The presence of shingle granules in the gutters can also be a good sign of roof wear and tear. You should also check all joists and rafters for signs of rot and make sure your rainwater system is running as it should without leaking onto the roof timbers.
Finally, you can protect your roof by taking simple proactive steps such as implementing effective moss control, installing bird comb protection, and so on. Depending on the age of your roof, it is probably always a safer option to replace your damaged roof with a new installation. However, do not just jump into conclusion. If necessary, you should employ the services of a professional roof installer to make a proper assessment of any potential roof issues and suggest the best way forward.