Retirement should be a reward for a lifetime of labor, but for far too many Americans, it's a time of belt-tightening and financial worries because they leave work before making sure they're financially prepared.

To avoid this mistake, watch for these three key signs you're not ready to retire yet. 

Professional older woman working at desk.

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1. You don't have a Social Security claiming strategy

Chances are good your Social Security benefits will be one of your most important sources of retirement income. Sadly, just 4% of retirees chose the optimum claiming strategy to maximize their lifetime Social Security income, according to a recent survey. It showed that retirees are missing out on an estimated $3.4 trillion in potential benefits because they didn't make the right choice about when to start their checks.

You don't want to add to that tab. Before claiming benefits, make sure you understand how your age affects the amount you (and your surviving spouse) will receive so you can decide what strategy makes sense for you. 

The formula can be complicated, but the bottom line is that if you claim benefits before full retirement age (66 to 67, depending on your birth year), you'll get a check that's up to 30% smaller than your standard benefit, But waiting until after your full retirement age enables you to raise your monthly check by up to 8% annually until age 70. Think through whether you'd prefer more smaller checks sooner or fewer larger ones later.  

2. You're not sure how much you can withdraw from your savings accounts

While Social Security will be one of your primary income sources as a retiree, it's not enough to live on without supplementary income. So if you're like most people and don't have a pension, you'll need money from savings. 

To make sure you have enough money in the short term and the long term, choose a safe withdrawal strategy and determine what amount of money it provides you. For example, many seniors follow the 4% rule, which says your savings shouldn't run short if you withdraw 4% of your account balance in your first year of retirement and simply increase withdrawals by inflation each year thereafter.

If you have $550,000 saved, this would mean you could withdraw $22,000. If that's not enough, you aren't ready to retire. You'll need to first find a way to reduce your budget and/or increase your income, perhaps by working part-time for a while. 

3. You don't have a plan to cover healthcare

According to a recent study from the Employee Benefit Research Institute, a senior couple could need as much as $325,000 to pay for out-of-pocket medical expenses in retirement, including Medicare premiums and the costs of prescriptions.

If you haven't estimated healthcare costs or taken steps to prepare for them, such as researching Medicare options and having dedicated savings set aside for your care, you aren't ready to leave the workforce. 

Sadly, once you stop working, it can be really hard to start again, especially with the country in a recession and jobs scarce. You don't want to get caught off guard by finding out too late that you weren't financially ready, so check these steps off your to-do list before giving your notice. You'll be glad you did when you can retire with confidence -- or if you discover before it's too late that you need more savings.