It's good to be vigilant and never totally stop worrying. Sometimes the catheter will suck against the venous tissue or be against a valve, restricting blood return upon aspiration, but still be a patent line.
My practice (especially when I'm unable to aspirate blood), is to palpate the tissue just beyond the IV catheter with two fingers as I flush. You can actually feel the saline flush thru the vein under your fingertips. Of course, one of the most tell tale signs is usually just watching the tissue around the site as you flush. Most times you can see the tissue expand with less than a 1/2 cc in most cases, and if you're not convinced even though it still looks good, then flush 5 cc's more if needed. Thirdly (and important to me), is watching the patient's reaction as I begin to flush, 95% of the time, the patient begins to grimmace and feel pain with a 1/2 cc or less if it's infiltrating.
Regardless if I'm confident in the line or not, I always return to check the site after starting fluids. I think that's just good practice, and will help your confidence in the long run. *** The worst thing is when you acually have a good line and send your patient to CT, and they blow the line with pressurized IV contrast. I hate that! :angryfire