The main things to look for when doing a personal roof inspection are signs of cracking caulk, rust spots, shingles that are curling or blistering, missing shingles, cracked vent pipes, moss or algae buildup and clogged gutters
Missing or loose shingles can usually just be replaced with new ones. Also be sure to check for missing nails or nails that are starting to pop out of the wood. You can easily hammer them back in.
If your roof is over 50 years old, you should be doing a yearly inspection for damage, leaks, peeling, damp spots, and water stains. These are early signs that the roof may need to be completely replaced, however if they can be reasonably be repaired, that it may be wise to just repair your roof, as that can delay the over head costs of replacing the entire roof for at least a few more years.
If you see signs of lots of wear and tear on your roof, more than a few shingles that are missing, caulking that is falling off or if your roof is leaking, these can be signs that its time to completely replace your roof all together. You should consult with a roofing company to get a quote on both repairing the roof or having the roof replaced completely, especially if the repair costs are going to add up to cost to the replacement cost of the roof.