The general standard one has to meet for filing any kind of factual assertion with a U.S. District Court is set out in Federal Rule of Civil Procedure 11(b)(3):
(b) Representations to the Court. By presenting to the court a
pleading, written motion, or other paper—whether by signing, filing,
submitting, or later advocating it—an attorney or unrepresented party
certifies that to the best of the person's knowledge, information, and
belief, formed after an inquiry reasonable under the circumstances:
(3) the factual contentions have evidentiary support or, if
specifically so identified, will likely have evidentiary support after
a reasonable opportunity for further investigation or discovery;
But how much pre-lawsuit investigation is appropriate/necessary before filing a patent infringement suit isn't particularly clear cut and depends a lot on the context of the specific situation. Traditionally, the bar has been pretty low - in general, all a complaint has to do is allege facts that, if true, state a claim for relief (i.e. a valid reason to sue someone). The Supreme Court raised the bar a bit in the last decade, requiring the allegations to at least be "plausible," but still not requiring "detailed factual allegations."
However, even the slightly raised standard is muted in the patent context because the Federal Rules also supply a "form complaint" for direct patent infringement (along with complaints for a variety of other types of actions), and the Supreme Court also held that complying with the requirements of these form complaints de-facto meets the necessary pleading requirements.
Paraphrasing Form 18, all you have technically have to do is state/allege that:
- My lawsuit arises under a federal statute (e.g. 35 U.S.C. 271);
- Patent No. X issued on Y date;
- I have/had an ownership interest in Patent No. X during period T; and
- Defendant D infringed my rights in Patent X during period T by making, using, selling, and/or importing into the United States a
doohickey W that embodies one or more claims of Patent No. X.
Therefore, give me money (and/or make D stop it).
As long as you can say all four of those things subject to the requirements of Rule 11, you can file a suit for patent infringement.
Obviously, the most contentious is element 4, that the defendant is infringing, but as long as you can make a decent argument that, based on what you know now, you believe that if you're given the opportunity to conduct discovery (i.e. dig into the details of what the defendant is doing and how its doing it), it's likely that you'll discover evidence that supports your allegation, then you've complied with all the legal requirements.
The standard for pleading indirect infringement is a bit higher. See @DennisCrouch's post on the subject.