Since he posted in 6.5 section, hoping a 6.5.
Turn on key and crack open the water in fuel drain and sample a pint for contamination. Let it settle for about an hour to see.
Replace the return line on front of the ip with clear tubing available at any hardware store. 1/4” diameter about 6” long piece. Get 2 screw style hose clamps for it. Then you can see fuel id flowing through the ip properly and look for a trail of bubbles. 1 trapped pea sized bubble is ok. But a trail means air being sucked in and needs fixing.
A fuel pressure gauge with the “T” at the ip inlet is your best friend. Knowing you have pressure needed for rhe starting right now in diagnosing obviously would help. But a dash mounted permanent gauge will tell you when the lift pump is dying or dead which quickly ruins the expensive injector pump too.
If you remove an injector line at the injector, does it spit any fuel while cranking?